Well, according to Randa (and according to Guruji and Lino, according to Randa), this is not supposed to happen. You are supposed to stay in the pose when Sharath (or whoever) tells you to inhale and exhale there. Then when he says "up", you simply lift up while still in the asana, and then jump back into chatvari position. In other words, the whole straightening the legs into dandasana thing, while a nice strengthening posture and fancy to look at and shit, is not necessary (and also possibly not "correct method", if you are the Ashtanga police :-)).
Well, I find this a bit hard to accept, probably because I have been doing this dandasana lifting thing for, like, forever, and find it a bit hard to let go of. But I'll see what I can do. Speaking of which, morning mysore is coming up in a few minutes here at the workshop's last session. We'll see how my attempt at no-lifting-up-in-dandasana goes. Will keep you posted on this :-)