Coincidence. While Rondo has been terrible lately, he really was not playing much better before he got the contract...teams just hadn't realized what he was doing yet, so he was being more effective.
Almost all season, Rondo has continued to play lazy defense (although he has gotten even lazier in the last few games, I will admit), and even lazier offense. On offense in particular, he has been relying way too much on the ability to find guys wide open for 3. In the first few games, it was easy, because teams were playing lax defense against the C's, which allowed Rondo to pick them apart. However, since teams have found that not only will he not shoot, but he has little interest in attacking the hoop this year, they have been not only sagging off of him, but they are not helping on him at all, and are instead staying glued to Ray and Paul on the perimeter.
So until Rondo makes an adjustment, they are going to continue to be struggling against good teams.