Mecanum drive puts added stresses on a belt because the wheels constantly change direction. It's not the acceleration or speed that's put on a belt that causes it to fatigue early -- usually it's repeated instantaneous direction changes. If you have robot-centric driving (i.e. the robot goes "forward" relative to its own direction) then the driver has direct control over how much stress it put on the belts. However, with field-centric driving (i.e. "forward" is always the same direction on the field, which is usually away from the driver's station) the computer calculates the vectors the wheels need and therefore the overall strategy and driving conditions control how much stress is put on the belts.
To help alleviate this in the future, you could make each toughbox transmission slidable so that sliding them with the belt in place will put tension on the belt. This eliminates the need for extra contact on the belt from external tensioners, thus improving the life of the belt. Or you could use a
stackerbox setup that does the same thing (note that there have been many improvements made to that first design since its release).