Seismic activity often plays a role in these kinds of incidents. The seafloor is over 8,000 ft deep in some areas that the cable traverses. Lots of stuff can happen at that depth. No submarines can go that deep so unless they announce at what depth it was disrupted speculation that it was man made activity is just that - speculation.
Are you sure about that? You may be thinking only of conventional attack subs, but don’t forget that subs have gone to the bottom of the Mariana Trench (depth - over 36,000 feet). It doesn’t take a huge conventional sub to cut a cable, nor does it even have to be manned.