yeah, it is happening with me too...Specially if you are not trainning your abs constantly...
For what i know, another problem is if you train too much the lower back (with deadlifts for instance) and dont do abs (well, im guilty of doing this, LOL). The abs and your lower back control the balance of your body, so if you dont train one of them while trainning the other too much you can create an inbalance...
In some stretches and some exercises my abs start to shake a lot and all my body start to shake...Im trainning abs constantly now and this is going away...