I noticed about two weeks ago, there a GE M250R2 HPS at a intersection near my house. The door was open on it with a bucket truck below, but no one around. Then later I had seen the door closed, but a week or so later, I saw it had been replaced with an AEL 115 250W HPS.

Does anyone know why they would replace a working HPS with a new fixture? To my knowledge it was working up to the replacement, and I could be wrong but I thought I saw it working, after the door had been closed. 

So I don't understand why. Although also, along that road, they have been replacing poles and lines but not the one at the fixture itself.

There could have been internal damage making it highly unsafe. Like cracked attachement to the pole or so. The fixture may still appear working, but would be threatening to fall down any moment.
This uses to be a result of someone hitting the pole in an accident. Even when the pole seems undamaged, the shock wave can smash the lantern very easily.