The unfortunate truth is that it is impossible to know for sure if it was unsustainably good shooting or if it was bad defense. Based on my eye test, I did not think the defense was all that bad. Jimmy Butler created most of the open looks. We should be able to adjust and tighten up the defense some. I am not sure about the "open" and "wide open" determinations. They were getting some good looks at 3s, but it didn't seem like there were that many wide open 3s.
So like many things, it is probably a combination of many things, unsustainable shooting for sure is likely a big part of it, defense also (both scheme and effort), and like I said, also Butler's ability to draw the defense and make the good pass (create).
The bottom line was that their supporting cast out played our supporting cast by quite a bit and that was not supposed to happen. Their supporting cast got the looks and knocked them down.