Attacking one monster versus attacking multiple monsters

Okay, so I've started playing a bit again on my level 150 Hero.

However, I'm a bit confused about why sometimes multiple monster attacks (shout, coma, and interpreted slash) will only attack one monster at a time. This seems to start randomly happening, and then randomly stops again after awhile.

Is this some mode that I'm enabling with a key combination and not realising it?

Sounds like you are using Enrage.

or triggering final attack, only two things i can think of.

Definitely Enrage. Final Attack is hardly even noticeable when mobbing, it fires so fast it seems like just one of the dozen or so hits you already landed.

Heidi, Enrage is not a "key combination", it's a single key you must have assigned the skill to. Check your skill window to see what its symbol looks like, and then find that on your keyboard layout (or possibly in a macro).

Enrage: Turns mob skills into single target skills but gives you a heavy damage boost.

Kinda like a corsair's homing beacon.

Yeah, it's enrage. I didn't realise it was a single target skill.