I have a 2007 pontiac Vibe and when it rains hard it will sometimes stall while you are driving it and then it proceeds to engage the starter and try and start. To add a little mystery you can pull the key out of the ignition and it will continue to try and start with the key out of the ignition. The only way to stop it is to disconnect the battery terminal
It needs service ASAP; point it in the right direction and have it drive itself to the nearest Poniac dealer service.