Mounting entities briefly prevents the camera from rotating.

If you mount an entity while rotating the camera, it freezes the camera very briefly. To reproduce place a minecart on an activated activator rail or place 2 minecarts on the exact same rail and enable cinematic camera. Then mount the minecart and spam right click so that you always mount the other minecart. While doing that rotate the camera and you will see that it briefly freezes it. Another way to reproduce this is by putting the command

ride @p mount @e[type=minecraft:eye_of_ender,limit=1]

into a repeating command block, throwing an eye of ender and the holding sneak, while turning the camera at the same time.