Apparently this is something that can just happen, provided the conditions are right. I had a bottle of water in my car, and even though it had frozen and thawed last week, it didn’t re-freeze when it got cold again, and even though it was -10, the bottle was still full of liquid water. All it took was a small shake, and the whole thing turned into slush. I actually recorded it too!
I think the craziest part was the fact that it didn’t all freeze, and some of it stayed liquid even when I shook it a bunch. I took it out of my car a couple hours later, and more of it had thawed instead of frozen. I drank some too and the liquid water felt almost warm, and the slush was definitely closer to 0 than -10, so it was way warmer than the rest of my car too. All in all, just a really weird experience.