It seems to have something to do with the trigger unlock condition. For example I have an unlock logic synched to a sector which in turn is synched to a trigger Blufor. So when Blufor captured the sector the trigger can be used to activate a respawn position and simultaneously trigger the next sector objective. It is this process that seems to causes the freeze up imo.
If all sectors are active at mission start the mission does not freeze up.
Btw when the sector is captured and the unlock triggers a new spawn point and new sector - the screen flashes back to the menu screen very quickly in both cases.