because of the ratio of each gear, the clutch has the most torque on the plates when you are accelerating hard in 5th gear, and the least when you are in 1st. Its very easy to have the engine wide open at 70 mph in 5th gear, hitting the peak of the HP curve, and pretty much impossible to hit the peak of the HP curve in 1st gear.
Another thing to check is to make sure your clutch cable isn't binding or hanging up.
Also when you are accelerating hard in 4th and 5th gear, make sure you are not pulling on the clutch level without realizing it, like putting your foot on the clutch pedal in a car... When you are riding a motorcycle WFO and you have a death grip on the handlebars, its easy to not realize you are pulling the clutch when you don't want to.