I can't speak for PC, but on Xbox it's not a Legendary thing. I've just done a quick test on Normal. After getting a checkpoint not far from the spawn trigger line, I repeatedly played on from it to see how many times an Elite spawned behind me. it happened 5 times out of 25, a 20% success rate. I suspect the spawning behaviour will be the same on the PC, simply because I can't think of any reason why they'd bother changing it. I know you said you'd never had an Elite spawning behind you, but I'd need to hear of a proper test to persuade me that the spawning differs on PC. I don't know how many times you've hit that spawn line, but you may've simply got lucky, or sometimes (amidst all the action) just failed to realize that an Elite spawned behind you. You could easily fail to realize if you head straight up the hallway, for example.