i don't normally block people but the reasons why i might are:

1) more often than not, if they blocked me first, i'm gonna block them back. call me petty but it pisses me off to catch random strays who have me blocked when i don't even know who they are so i just block them back to avoid seeing it. applies to oomfs that blocked me for whatever reason as well, i'm just trying to keep my space comfortable and pleasant for me and for my own peace of mind.
2) if they actively participated in a witch hunt / dogpiling against me, specifically if they refused to listen to me or let me explain and were comfortable with spreading around misinformation and publicly speaking ill of me (and MORE specifically if they used to be close with me and randomly decided to join a hate train), i will block them. if there's a call-out or 'expose' post on me and i see people on the replies talking about it, then doubling down when i reply respectfully to try and explain what really happened, i'm most likely gonna block them because to me it just speaks of a very close-minded, obtuse way to move through life. i appreciate people who can acknowledge other perspectives that are not their own and if i realize that's not the case, i will block.
3) if they think they can get a point across by insulting me or being just overall rude and disrespectful. i've had it said about me a couple times before that i 'refuse to admit when i've done something wrong and block people who call me out on it' because i do, in fact, block people who talk to me disrespectfully or insult me, even if they're right for calling me out. any person that calls me out respectfully will always get a major thank you and appreciation for me, but a person who calls me out for the same reason (that might be right!) but is rude and crass toward me will be blocked.
3) anyone i find out has talked badly about me without me knowing or is friends with people who hate me or actively speak badly about me, i'm probably gonna block (if i can be bothered to). it's nothing personal, you might not have said anything bad about me, but i just don't like feeling like these people that dislike me have a 'mole' or someone lurking on my account who can tell them what i'm up to just to have excuses to bring me down even further.

i think those are the primary reasons why i'd block someone, as i don't normally block people at all. might as well add, i recently had some problems involving a bunch of people where i found out there's a small friend group involving people that all deeply dislike me and were talking ill of me for no apparent reason. out of this bunch i already had most of them blocked, so i can't be bothered to care about them, but there were one or two that were interactive mutuals with me and whom i decided to block because it felt awfully two-faced and hypocritical and i didn't feel neither safe nor comfortable knowing they'd said stuff about me while pretending we got along. i don't know who you are, you might have been one of these people; otherwise, i'm not sure.

if you need a genuine explanation as to why i blocked you specifically, let me know ur user and i'll dm you.