The Bug
Minecarts with TNT shake rapidly before exploding when coming into contact with fire or lava. Other minecart types also shake when on fire, but it's less noticeable since they disappear quickly (whereas TNT minecarts linger for several seconds before exploding).
How to reproduce
Place a TNT minecart on a rail.
Light fire on one end of the rail and push the minecart into the fire.
Affects 1.16-rc1