I'm betting it is a clog and that it is most likely from tree roots. To find out if it is a clog and if it is roots, run an auger into the drain and, when you get to where the tree is, you would hit some resistance. Try to break the roots loose and pull them out.
If tree roots are the problem, buy some pellet salt like you would put in a water softener. Put a cup of the salt pellets in the toilet once a week and flush it down. Those pelltes will collect on any root formation, lay there and slowly dissolve, killing the roots. If you keep them killed they can't accumulate enough to make a blockage.
Tree roots in the drain mean there is a small leak in the drain; bermuda grass is just as wicked as any tree.