It can be caused by wearing headphones and playing music loud enough and long enough, which I had to do extensively in order to do my work and focus, because if I didn't have them on there was often 4-6 conversations that I could hear from my desk at various times throughout the day (and not too uncommon there'd be 2-3 different conversations literally next to and across from my desk, it was apparently a hotspot for BAs and scrum masters to congregate and chat for 45+ minutes at a time. I tried to get my assigned desk changed but had little luck with that).

I think at one point I got noise cancelling headphones and that helped a bit more, but I still had to play music at a certain level to cover the sound. Don't quite remember anymore though since it's been four years since I was in that situation.

I'm sure not every open office is like that. In particular the one at my current job, the one time I went in, was significantly quieter. Although the office was only a third full as well, since things changed a lot since the pandemic started for how many people show up to the office (I work remote almost exclusively now).

To be clear, I don't know for certain it caused it, it just seems the most likely culprit out of the possible causes in my life.