In my case, most users I've blocked were for shitty behavior. I have zero tolerance for that. If someone speaks to me or others in a way they wouldn’t dare to in real life, I block them instantly. I don’t mind disagreements or different opinions, but I won’t waste time with mean or nasty people.
The second most common reason is shallow, one-sentence replies only meant to fish for upvotes without adding value to the discussion. Jokes are fine, but if you're just echoing what's popular for applause, you’re gone. Nothing wrong with that per-se, but I’m more interested in serious conversations.
Lastly, there are petty reasons. For example, overused phrases like 'fuck around and find out' will get you blocked too.