It would have been something you said in the feedback that caused removal, although by asking seller to cancel sale, to which you agreed, might have been taken into account.
Cancelling as out of stock gets sellers a defect, so by asking you to agree to cancel it let seller off the hook.
I always suggest when seller does not honour a sale that negative feedback is left, just saying “seller did not honour sale” or “Unsatisfactory transaction”. Neither of these would be removed, but ebay encourages long feedback the problem being that the more said the more likely that it will provide an excuse for removal, especially with negative feedback. Just mentioning ebay, cases etc. is sufficient for removal, so if the need arises again, just keep it brief.