non-flying kamikaze aliens only explode on impact when they're having at least a height of 450 or somwhere in that vicinity, perhaps in order to touch the player's torso and not just the legs. I believe that's the reason why the Lookers in Marathon 1 are flying, although their graphics suggest that they're crawling on the floor.
There's a sample map below with virtually normal looking assimilated bobs but with a physical height of 350.
kamikaze problem test map
This is fixed in SVN r4750 (film profile 1.1). If the monster can't float to the desired height, it's allowed to explode at the target's feet, as long as it's not on lower ground.