Whenever I approach an Imperial camp the Imperial soldiers will draw their weapons and give the usual, "You need to leave!" spiel and attack me if I linger too long. This would be expected if I had joined the Stormcloaks, but my character hasn't touched the Civil War yet, nor done anything that would piss off the Imps in general.

I've had this two playthroughs in a row, and it appears that this hostility is triggered by "Season Unending", but I don't know how.

Some notes:
-Stormcloak camps are unaffected.
-Both times I did the "Season Unending" quest the treaty was considered fair.
-Joining the Legion solves this issue.