I didn't often enter those unless absolutely needed. It just used to happen for no apparent reason that I could fathom. The NG VNAV descent profiles were very unreliable and frequently had to be ignored. The 747-400 VNAV used to be almost perfect, 15 years later the 737NG VNAV profile could be anything up to 8000' out and always needed to be checked by mental rule of thumb all the way down. Never to be trusted without checking.