I don't know if this will just create more confusion, but I had started experiencing this problem over the last couple of days after uploading a lot of songs to my shared Apple Music account (not iTunes Match, but similar). A random selection of those uploaded songs filled up my iPhone and drained my battery overnight. They were immediately re-downloaded after I deleted them as others have reported, even when I had turned off iCloud Music Library. I called Apple Support, who talked me through a number of things. By the end of the call, my downloads had stopped but we don't know why. The two things that I did during the call that might have made a difference were:
1) In the Store tab of my iTunes preferences on my laptop, which was signed into my Apple Music account, I unchecked "Always check for available downloads". The Apple support thought that should only be relevant for my Mac, not my iPhone, but I thought I would mention it.
2) In the Music app on my iPhone, a bar at the top said "Downloading a song" - it had previously said "Downloading 33 songs" or some other number. I hadn't realized that pressing that bar opens a dialog where you can cancel a download - someone earlier in the thread mentioned this, but I had missed it.. When I cancelled one song, another immediately replaced it. However, after clicking the "cancel" button a few times, the downloads seemed to stop. I suppose that it's possible that the iPhone had a cached list of songs it wanted to download, whatever the other settings, and clicking "cancel" multiple times cleared the cache.
I had done so many other things during the call, that I can't guarantee either of these were what fixed my problem, but they could be worth trying.