Creepers which land near the player explode immediately

I have arranged for mobs in a mob grinder to fall a distance of 21 blocks exactly, landing directly in front of me so that I can deliver the killing blow.
However, the system causes creepers to explode within 1/4 of a second of landing. This is before they have had a line of sight to me for 1/4 second, and before they have been in fuse-lighting range for 1/2 second.
While many creepers explode with a normal fuse, This happens about 50% of the time, with no obvious factors deciding which behavior they will choose. However, they never explode when I am not present at the landing location.

This did not seem to happen a few weeks ago, perhaps because of the increase in local difficulty since then?