What's going on when a -ku godan verb like "叩く" gets conjugated to "叩かれ"?

Like what’s the exact conjugation breakdown going on there? I’ve spent days scouring various things and I can’t for the life of me figure out the exact breakdown causing a "かれ".

I’m not leaving off any conjugation bits, so it’s definitely not me misreading something like 叩かれた.

If you could explain what’s going on in general terms rather than specifically -ku verbs, that might be more helpful, but I’m at my wits’ end and will honestly take anything at this point.

力いっぱい背中を叩かれ、大介は思わずよろめいた。

Two friends, the person strongly hitting 大介 on the back is a friend trying to cheer him up. He did it without warning, but it’s not aggressive. The sentence is narrative and describing the situation, non-spoken.