The term actually makes complete sense.
Torque is any rotational force applied to an object. When you throw a disc you're giving it torque in order to get it to spin on a level plane. For a hyzer release, that plane is tilted away from the thrower and for anhyzers it's tiled towards the thrower.
It's nearly impossible to apply torque only on that one single plane. Just because the disc doesn't come out spinning sideways doesn't mean that there wasn't some measure of torque applied in that axis. All of this extra torque is applying rotational force off the intended axis. Thus, it is Off Axis Torque.
Imagine spinning a top on your desk. If you spin it absolutely perfectly, it will stay perfectly straight up and down at the start with the only motion being around it's axis. But that never happens, there is always a bit of wobble, this is because of the torque being applied in an infinite number of other axes. Too much of this OAT and the top will simply crash to the ground. With a minimal amount, the "On-axis" torque will dominate and keep the top standing.