It doesn't seem to make much sense when you're in their faction, and they already know Harkon gave you the power. The strange thing is that when I remained in vampire lord form after the little lesson with Harkon, they didn't mind. But the actual transformation seems to turn everyone hostile.

How the hell did you manage to transform into a vampire lord in the first place? I just bend down and thats in, no transformation, am I missing something?

thats weird. i hope they didnt do that on purpose. if it was my castle id have everyone in lord form 24/7 cuz its badass lookin

It hasn't happened again. I guess it was just a fluke or something.