SP/Co-op will sometimes CTD for no apparent reason, even on maps that normally don't crash.
Every CTD has a reason for CTDing,an error "should" showup in windowed mode,if not you must run in debugger mode
And dont fall into the misconception,that "i was doing this or that at the moment"
Many CTD's are caused by bots,drop bot numbers to 1,and theres a very good chance,the game will play longer before CTDing
Note,some CTD's can be caused by the graphics card resetting itself,this will only happen when there is too much action onscreen,reducing bot numbers will prevent the above type of CTD