Generally (if not always) I am holding down the left alt button with activity happening around me such as multiple AI, multiple players (3-5), or driving a vehicle/flying an aircraft. It doesn't happen all the time, but when it does happen, its usually at an inopportune time as I am trying to complete a mission with other players, or flying over a "populated" area of the map.
I crashed yesterday driving a BDRM towing a Hilux pickup. I found myself "stuck" on a steep hill incline. As I tried to find an angle to get off of the incline, the game crashed on me.