Hiya,
It's very hard to say, it could be something to do with your machine's setup (patches etc) or it could be a problem with the game data or simply a bug in ScummVM AGA itself.
You could try the same game with a 'clean' WB install in WinUAE and see if it falls over in the same place.
I take it this crash is repeatable?
I also had a bug with Broken Sword 2 which I could never get to the bottom of....