Did you just get a new keg delivery? Warm kegs and kegs that have been moved long distances can cause foam.
Did you just hook up a new keg? If so did you bled all the air out of the drop line when you were tapping the new keg? (if air gets into the line running to the tower it will cause air pockets in the line which will cause foaming when you try and pour.)
Are the seals on your coupler or keg stem damaged? Damage can lead to air getting into the line which causes foam.