I mostly use iris.to (self hosted), but also Damus (iOS) and snort.social. This reply is via iris.to.
This may be the client trying to tag the folks in the thread. I see "replying to:" at the top of my last post. I'm guessing that's due to the p tags you are asking about.
I'm only two weeks into nostr, so if there's something I'm doing wrong, please let me know...