The lemming is definitely exploding at a lower place than normal, triggered by letting a lemming fall a distance that would cause it to splat. The act of an exploding lemming overwrites a lot of actions. If a lemming begins to enter the home and during the animation the fuse time expires, the animation will reset because the exploding animation will kick in, then the game will detect that the lemming has reached the exit, so plays the animation again.
The variable lemming fall distance is a feature. On each map, you can set this maximum fall distance. The default value is 56 pixels. The sturdiest lemmings are the ones in "Just Dig", who can fall a distance of 72 pixels before splatting (in fact, this is the only level where the fall distance is above the default value). The most fragile lemmings, on the other hand, are in the following level "Only Floaters Can Survive This", where a fall of just 32 pixels will kill them.