Well, hard to be absolutely certain without a photo, so all I can do is give the usual causes - windy conditions, hot sun at midday, or, in your case, it needs potting up into something bigger and isn't getting suffcient water because the pot's now too small. Dieback can be another cause, but its impossible to say for sure.

Though this is the time of year when acer leaves shrivel and fall off. I'm looking out of our sitting room window at one of ours that is looking sad and sorry for itself right now, but it is only normal leaf fall.