Thought unlikely, the Dawnguard could have spawned and attacked the Castle when you werent there, though they should have spawned near you? That has happened to me before, I have returned to Dawnguards running haywire in the castle while all the Vampires chased them.
However, I'm on Xbox 360, maybe they both do that?
It is simply a scripting error if something like that happened.
most definitely not - it is some sort of an issue, as she (and the beastlord whatever character) die upon entering the castle sometimes (and even if i play on pc it doesn't help as they die again next time) and that leaves the essential vampires on their knees (suggesting all characters receive damage). This ruined my vampire playthrough.
She died on my game too, 360. I think I know when it happened:
Harkon calls on everyone to find a Moth Priest.
A few vampires run out the front gate.
They come back.
What I think happened was the same glitch that happens in Kynesgrove sometimes; her spawn was set too high, she was not essential, but Protected, like spouses after marriage. She fell, a Dawnguard shot her in descent, and she hit the ground with not much health, and was shot to death.
She died in my game when I dropped a platter on the floor after accidentally picking it up. One of the male vampires (I forget who) said something like, "Look what I found!" and she goes, "I saw it first!" or the like. I've also seen the same dialogue between a wife and a steward at one of the Hearthfire houses in a youtube video. The difference is, Hestla isn't essential, and also she glitched out pretty bad during the fight. She walked over to hit the other vampire once, then immediately turned back aground and started taking tiny, janky little steps away while he slashed at her furiously.
Since I was thinking, "oh, it's that one fight I saw on youtube but with different people," I walked off to go kill Harkon. When I came back, she was dead in the smithing area. Welp, that's one less smithing vendor for me because it already took two different attempts to kill Harkon due to HIS bugs.