The out-of-control part of the Chinese rocket Long March 5B is expected to re-enter the atmosphere of the earth and crash on the earth in a few hours. (Today is May 8th, 2021) At the moment, it is revolving around the earth in an elliptical orbit and completes a revolution in 90 minutes. Since April 29th, 2021 it has gone around the earth more than 100 times and the size of the orbit is getting smaller and smaller.
Why can't this rocket keep revolving around the earth until it is completely burnt in the atmosphere or hits the skyscrapers (in a path parallel to the earth surface and not perpendicular to it, so to speak)? What laws of physics would cause its re-entry into the atmosphere? It is losing altitude but what prevents it from losing altitude gradually and continuously (without a sudden change in its path)? It is already experiencing the gravitational pull of the earth but what is supposed to cause its 'sudden' downfall towards the earth and crashing on the earth?