Zookiemike, I assume you are talking about off roading.
I know it can be because of wheel pressures.  Especially in sand.  If you have too high a pressure your wheel will dig (go down) then clime out (up) and dig again.
I've wheeled sand for 20 years.  My brother-in-law had a chev. 3/4 ton with new tires.  No matter how hard I tried, he would not air down past 15psi.  When he went onto the dunes it wheel hopped so bad that my sister (his wife) would not ride with him. ÂÂ
As for other conditions, it's just a part of wheeling.  If you tires are too high for the soil conditions, the wheels will again dig in and clime out causing the truck to hop.
It's not the same axil hop that someone with leaf springs.  Wheel hop with leaf springs is when the torque on the axil and wheels cause the leaf springs to twist until there's so much tention they spring back.  Thus causing wheel hop.
Note:  Wheel hop is the number one reason for breaking parts!  The tire or axil goes up into the air, with no resistance the tire will "spin up or spin faster" (this is more prevalent with open diffs).  Then the tire comes down and make contact with the ground.  Now if the tire grabs instead of spining, all that force that was spining the tire (torque/power) is transfered to the drive train.  Then you are most likely hear the sound of axiles braking. ÂÂ
