What could be causing my toilet to back up?
It sounds like something is in the trap of the toilet. To be positive, you can run a camera through it, or alternatively, remove the toilet and pour a large bucket (larger than a flush) of water down the exposed drain - if it drains quickly and does not backup, you know the issue is with the toilet.  If it backs up then you know the issue is with the drain (or vent possibly)

If the drain is the problem then the best bet is to have a plumber come run a camera down it to determine where the problem is (from the question you said you "cleared" it, I assume this means running an snake/auger down it)  It could be an object is stuck, or the drain could be collapsed, etc.

If it is the toilet then you need to and remove whatever is stuck in the trap. Sometimes objects like toothbrushes can get really stuck and the only option is to replace the toilet.