1) They are miles ahead in the polls and think they can increase their majority (as in 2017)
2) They have suffered a humiliating defeat or scandal.
Will losing control of some councils result in a General Election? If you think so then Boris has a bridge he can sell you.
- TM getting wind of a coup and calling a GE just to spite her party for stabbing her in the back (unlikely, and *very* unlikely to happen before #Brexit)