I was committing a change on the main repo (which contains a few submodules) and was typing up the commit message. I decided that I wanted to change my message, and used the CMD+delete to clear the line, but that key combination does something else in Sourcetree. I often make this mistake, but its usually benign (just throws a prompt asking if want to back out of committing). However, in this case, it threw a couple of warning messages, and deleted my submodule (I now had a change in .gitmodules showing the submodule was deleted, and the actual submodule folder removed). At this point I have now lost a whole days worth of work.
Luckily after some searching, I was able to recover 90% of my work from PyCharm's local history, but not before I nearly had a panic attack.
Sourcetree usually crashes on my about once every day or 2, which I think may be memory issues, but never anything like this. I unfortunately do not have the error messages that popped up, nor did I choose to send the error report, so there isn't really much I can contribute to the actual error that occurred, other than it crashed after hitting CMD+delete in the commit message text box.