My guess is that you're playing GigaPiano II, which has embedded GigaPulse for up/down pedal samples. When you press the pedal, it has to switch things around. If your processor is already at its limits, then the pedal can cause a crash.
What does your CPU meter say before the crash occurs? Does this happen with, say, the Post Bos290 demo piano (which doesn't have GigaPulse)?