Settlement security respawn

[Tiviss Sar]

Ok, I was sloooowly biting off turrets(high security base), shooting till my shields were almost dead, then hiding below their line of sight to regenerate and repeat. After half an hour I managed to destroy most of them, leaving only two standing, when suddenly the base decided to reset and rebuild the whole thing. I managed to run away with 40% hull (Vette) and a sad/    face. Is this normal? I get they should respawn after some time, but i was thinking it would be more like several hours, not minutes. o7