Ditto Undisguised Out in the Open?

I wish I'd taken a picture... I saw a ditto just chillin' totally undisguised, so I caught it. You know how with a disguised ditto, it does the whole "Oh, what's this? It was actually a ditto!" thing? Well after I caught this undisguised ditto, it did that and turned into a Zorua instead!

I tried briefly googling this phenomenon and found a single mention of somebody seeing an untransformed ditto in the wild, but they didn't catch it, and nobody in the comments had an explanation. What in the world just happened?