Sorry call me stupid, but why are aircraft heading to the engine bay to hold once they filled up with passengers and left the stand?
If they have a slot delay they remote hold because it shows to the pax that they were ready and it's not their fault that they have been delayed. I believe it also affects their stats because pushed back on time.
Having said that, occasionally they are asked to remote hold by the airport to free up stands. But when I was there it was usually the former why they went to the run up bay.