Most common would be minimum separation from preceeding traffic providing wake turbulence hasn't already precipitated the g/a switches and then that traffic not clearing the runway. All of the above from the esteemed Heathrow Director.
Any number of config warnings.
Weather related, cross wind exceeding limits, windshear, microbursts etc
Runway incursions either by rogue animals, vehicles or indeed other aircraft. Birds (large visible flocks) if seen or advised can precipitate a g/a.
Any number of connotations could cause you to discontinue the approach, it will be PIC or ATCO initiated dependant on the circumstances.