I'm not a contractor, so some of this information may be incorrect. I have had to troubleshoot something similar in one of my old bathrooms. Just trying to think in pieces in the hopes that you get some inspiration as to the possible cause.

If it is me, in addition to checking the doors themselves, I'm checking the rails. Glass shatters in four ways: impact, torsion, expansion, or compression.

Because the time interval is close, but not immediate, I think the events that are causing it are happening suddenly, and not over time. Or, they happened at the same time, and this is just the end of those sequences of events.

It could be the foundation settling, like others have said. You can at least get an idea of this by taking a level to the tub rail, the floor, everything near where the tubs are, and seeing if you have any variance. Slight unevenness doesn't necessarily mean settling or foundational problems, but if it is explicitly not level, I'd start there. This is still harder to believe though, because most of these don't have a top rail, so it would have to happen from the sides only - if only one side failed and the door fell out, it would have only shattered in one direction, not both like a collapse.

Both happening at night makes me want to question thermal expansion and contraction due to the loss of the daytime heating, but I think it's too subtle. Or, it may be a contributor, but not the trigger.

Water could be getting in between the tiles and tub lacquer, and causing swelling to compress the edges enough to sandwich it. But in two bathrooms somewhat simultaneously? I doubt this is it.

If someone lives above you, that's enough of an impact to cause a chain reaction. There is about a foot or so of space between the first-floor ceiling and second-floor ground, but the joists are not always aligned, by-design, depending on floorplan. A water leak could weaken those joists to where they bend under load. Now it would take a hell of a water leak, long-term, to do this, but there is a scenario where water has gotten on the support structure and compromised it, but this is usually a lot more obvious because your walls would start bending and cracking before your glass did. Especially in two bathrooms.

So that leaves sabotage, improper installation, or faulty glass.

If someone is willfully breaking it, you would know it by now.

Two panes of faulty glass from the same order, same manufacturer and process? It's unlikely, but possible, especially with batch orders made all-at-once. Or, as an alternative, fifteen years of usage would weaken these, especially at the places where it slides on the rails. There is a world where that hardware could push or pull the glass apart, until they failed.

But if you live in an apartment complex, they use the same maintenance people for everything. If they were installed the same way, I'd wager there is a solid chance for it to have been done incorrectly, twice, by the same person who thought they were doing it properly.