Don't know why your post got deleted, but it isn't a big deal even if you got merited for it--and for that merit you got, be happy. Someone must have reported your post for whatever reason and the mods agreed with the report. It happens to most users here, I think. I've had plenty of posts deleted, and it's just something to accept and move on from.
You posted in a thread that was at least two years old.
Ah, I missed that. Necrobumping is a sin on this forum. That's why your post was deleted, and it's not abnormal. You were answering a question that the OP of that thread has no doubt found and gone away. He ought to have locked the thread, and I'm not sure why you bumped it.