A short jump or one with a steep lip at the top. On either one, your front wheel has already left the ground the rear end gets kicked upward by the jump. With the front wheel in the air, when therear gets kicked up, the front goes down. The best method for dealing with that is to be accelerating hard all the way over the jump.
sometimes jumps that don't have kickers develop them when they get worn. The lip is packed real hard and then for 2-3 feet below it the dirt gets thrown out leaving something that as Gomer said get's more steep than originally intended. But yeah, like pretty much every problem in MX, more gas is your friend! :) When in doubt...twist the wrist.
*disclaimer: this advice is coming from a never-was with a foot full of metal!
For forgiving jumps the incline should be 3' run up for every 1' in height. SX type jumps are more along the lines of 2' run up for every 1' in height. Which ever you chose, it is always an easier jump if yoou get both wheels on the face before lift off.