You used the "fixed" term in you first post. Something I had never heard or read before in this context.
But anyway, I suppose there is just a difference in terms and understanding depending on the
respective countries.
There are even several other ways to perform the procedure:
Ligature of the oviduct with uterus intact
Ligature of the oviduct with uterus removal
Removal of ovaries with uterus intact
Removal of ovaries and uterus...
... just to name a few ;)
The point is to impede reproduction but otherwise keep the natural behavior, hormones etc. intact so the bitch will not become "leaky" or sparsely coated or prone to other inconveniences related to hormonal deficiency.
And removing the uterus prevents the otherwise very commonly and often fatal...
If I understand right she is only fixed and not neutered. This means her ovaries are still working which might be the reason for the aggressive behaviour.
Another reason could be a tumor. Our neighbors had a Dogo Argentino bitch which suddenly turned very aggressive - long story short: she...