Can someone explain the frenzy feature for path of the berserker

This feature makes no sense to me, is it like hunters mark and triggers on all attacks or just the first one.

It's worded in kind of a strange way. Read in a very literal way, it applies to all attacks against the first target you attack on a turn. So, if you make multiple attacks against the same target on the same turn, it'll apply to all of them, but if you make two attacks against different targets, it only applies to the first one.

Assuming that's the intended interpretation, I think the idea is that it rewards you for focusing your attacks on a single target — which makes thematic sense — rather than spreading them around.