If the thread of the nut is fine the spindle thread must be damaged. Could be due to tightening it too much (impact wrench) in the past. Typically the nut would get damaged first, did you check that?
Not sure if hitting something could cause that, if it's plants I'd say rather not. I am hitting branches in the woods and wooden boards lying around all the time and big stones (cutting the neighbours grass

) and recently concrete kerb stones (ahem..., yes, I know

) and nothing came loose. I changed the blades two days ago, see also here:
Link RC48 blade change