@petehall I don't think they are connected as others, like me, saw different shifts on those dates. My site went up on the 23rd, down March 30, down April 3. Getting completely different traffic (different keywords) each shift.
If that's Panda, it's a funny way to go about it. No, I think it's one of the other 50 or so changes they'll make this month like they have been since November.