AFAIK, it’s partly because they drop an explosive charge on the thin roof armour, and partly because drones dropping explosives are often accurate enough to do so into an open hatch, in which case the vehicle might as well be unarmoured:
Oddly, though, there don’t seem to be any Russian soldiers around despite this video showing a tank that was clearly being recovered, but still, it illustrates drones can drop simple explosives into open hatches.
Top-attack ATGMs like late-model TOW and Javelin cause similar problems. Here is a video of an American test of a TOW missile with top-attack warhead (that would be a TOW-2B, off the top of my head) against a T-72:
You can see the missile fly a few metres
above the tank when it detonates. This fires a charge downward into the roof armour, and … well … the result speaks for itself. Javelin is even worse, that dives headlong down into the tank, as you can see in this test firing:
The point of these “cope cages” (the mesh roofs) and “turtle tanks” (the ones that look like mobile sheds) is almost certainly to try and detonate warheads before they reach the actual tank. It proved totally ineffective against top-attack missiles, because those probably strike at such velocity that they punch through before detonating against the main armour anyway. The charges dropped by drones, though, should be easy enough to catch and detonate relatively safely — at the very least, on the
outside of the vehicle. People derided the Russian cope cages two years ago, but oddly, nobody seems to when the Israelis quickly put them onto their vehicles as well in Gaza:
AFAIK, the “turtle tanks” seem to be limited mostly to tanks used as artillery, mine-clearing, etc. where turret rotation isn’t all that important, but keeping drone attacks off the main armour, is.
