Was slaying skeletons, was assisting the group, then this happens:

<PROMPT>
Dramdor closes his eyes for a moment and nods at Althrsis.
Althrsis closes his eyes with a look of concentration for a moment.
Althrsis's eyes blaze with holy fervor as his wounds heal before you.
A skeletal warrior is writhing in agony.

<PROMPT>
A skeletal warrior's leveraged kick maims Althrsis!
Althrsis is sent sprawling backwards!
A lesser archangel rescues you from a skeletal warrior!
A lesser archangel parries your cleave.
Your cleave mauls a lesser archangel.
A skeletal warrior's stab maims Althrsis!
A skeletal warrior's stab devastates Althrsis!
A lesser archangel has a few scratches.

How did I end up fighting the angel?

Thanks in advance.

Did your angel do something to draw your group into combat with it (e.g. pillar), before it rescued you from the skeleton? If that rescued you out, you might have then assisted against the angel.

n/t

Generally after a pillar, where it hit groupmates.