Interesting thing about the Russian tanks I never really realized. The ammo is basically stored in a large ring underneath the crew. That's fine when the tank is hit straight on, because the armor is usually thick enough to stop incoming rounds. However, the armor is thinner on the sides, and the top of the turret offers almost no protection. I guess that's because the T72s were designed at a time when the main threat to tanks were other tanks, which of course can't hit the top of a turret.
So what happens with a Javelin hits from above or an NLAW detonates from above, the blast goes down and into the crew compartment. If someone manages to survive the initial hit, then they soon get vaporized by the much bigger secondary explosion, which occurs when that ring of ammo underneath the crew cooks off and blows up. That's why exploding Russian tanks also have their turrets fly sky high.
From what I understand, the Ukrainian forces also rely heavily on T72 tanks, but the big difference is that they face other tanks and rocket propelled grenades, not anti-tank missiles like the Russians, and so the Ukrainian tanks are in that sense not as vulnerable as the Russian ones