My take on this is to question timing more than the motive. Yes, this could easily be retribution. But let us ask the question, just how much do you need for an indictment of a famously sleazy New Jersey operator like Menendez? The illegal trips, special pleading, etc. have been in the news for many months.
Menendez's predecessor Bob Torricelli, was run out of office for similar things. Corruption comes with the job description in my former home state and frankly, getting to high legislative office there is all but impossible for a morally upright human. Any assiduous US Attorney can indict any one of these characters at more or less at any time.
So my question is, how likely is it that Eric Holder's minions just happened to find something that pushed them over the edge this week? We will see if this goes to trial and Menedez's counsel plays the I was the target of selective prosecution card. I'm not sure a jury will find the "I gave up my career for my love of Israel" line very convincing. But then, this will be a New Jersey jury who will be no better than they ought to be.
What this all means is that we will have an opportunity to see a real two scorpions in a bottle trial, with tainted parties on both sides.
I have only tangential connections with the good Senator. Once, long ago I was at a party put on by one of his aides. It happened that a US helicopter went down in Central America while he to was in the air in the region. His staff feared the worst for a time until they learned that the copter was a US military one assisting in our fight against the communist rebels in El Salvador. According to one of the aides when told about it, Menendez referred to the soldiers as "Those bastards." I'm not sure if the soldiers lived or not.
This is also the senator who hired an illegal alien sexual predator as an aide.
I also remember when a well liked federally funded community action program was taken over by persons loyal to the Senator.
I'll let this quote from the Sunshine Foundation's site fill in the details.
Relationship with non-profit group
In early September 2006, federal authorities delivered a subpoena to the North Hudson County Action Corp., a non-profit organization in Union City, NJ. Specifically, they sought records of a rental deal between the agency and Menendez.
In 1983, Menendez purchased a three-story brick house on a residential street in Union City for $92,000. He and his family lived there briefly, but ultimately moved out and began renting it as a commercial property. In 1994, he entered a rental agreement with the North Hudson group, who planned to use the home for administrative offices. Menendez charged them $3,100 per month, and later raised the figure to $4,000 per month.
During a nine-year period in which he served Hudson County in the House, Menendez received over $300,000 from the group in rent payments. As a congressman, Menendez often helped secure funds for the non-profit organization. In 1998, Menendez helped the agency win designation as a federally qualified health care center, a title that made it eligible for federal health care grants. In the eight years that followed this designation, the agency collected $9.6 million in federal grants. In 2001, the group honored Menendez as it’s “man of the year” and named a pavilion after him.
Menendez argued that before renting out the house in 1994, he obtained clearance from the House Ethics Committee. He has stated that there was nothing improper about his actions because he never negotiated directly with agency officials and the organization paid slightly below-market rent for the house. In 2003, Menendez sold the house for $450,000. The new owner continued to rent the house to the North Hudson Community Action Corp.; at a rate of $3,700 per month.  
Currently, Menendez is under investigation by the Newark, NJ Attorney’s Office and the Department of Justice for collecting rent from an anti-poverty group that received funding from earmarks proposed by Menendez. The group, the North Hudson Community Action Corporation, is run by one of Menendez's donors. The federal investigation has broadened to encompass Sen. Menendez’s relationship with his former chief of staff, lobbyist Kay LiCausi, to determine whether Menendez improperly helped her or her clients win contracts and federal funding. Menendez has not reported any legal fees in 2009.
Hiring controversy with top advisor
In September of 2006, a tape emerged of Donald Scarinci - Menendez's chilhood friend and closest political advisor - pressuring a Union City, NJ psychiatrist with several contracts with the prison system to hire an additional physician known to Scarinci. In the secretly recorded conversation, Scarinci told the physician that doing so would be a favor to Menendez, then a member of the U.S. House of Representatives. When the tapes content became public, Menendez severed ties with Scarinci, claiming that he had used his name and influence without his knowledge and committed a serious breach of good judgment.