good question and 99% of meatheads don't even know the answer.. i attribute it to this:
1. rapid blood flow going into the muscle
2. mineral depletion
pumps aren't a bad thing, the problem becomes when they get so bad that they cause you to stop your workout. the way you combat it is simple, lower the dosage and run n2guard which has taurine and other minerals. stay hydrated too. you can hit me up for a code