Lots of possible reasons. The one that has given me the most trouble over the years is wear in the throttle shaft & bushings at the front of the front carb. With the throttle linkage always pulling downward (to overcome the return springs), both the shaft & bushing wear until they let the shaft droop lower. So if you close the throttle gently (ie normal driving), the edge of the throttle plate hits the bottom of the bore and binds. But if you blip the throttle, the plate levers the shaft upwards and closes against the stop.
Another one that was hard to find was wear in the pivot of the bellcrank allowing the bellcrank to tip far enough that it was dragging against the bolt head of the bolt that holds the pivot to the manifold. I neglected to take a picture at the time, but this photo shows the problem spot (along with my solution at the time, which was to cut down the bolt head for more clearance). (I've since reworked the pivot to have less slop.)
Yet another problem was caused (more or less) by replacing the nylon bushings that carry the linkage left to right. The new bushings were tight enough that they would bind when they got hot. (Probably doesn't help that my linkage shaft is not perfectly straight, so the bushings have to move a little bit as the shaft turns.) Fortunately, they seem to be wearing in enough that now I only get binding when underhood temperatures are very high. Should be OK with another few months of driving.