When you turn your wheels, the built-in caster will force one end of the axle to rise, and the other to drop. This alters the static loading of the springs significantly. This change in loading will force the springs to flex, one in relative compression, the other in relative rebound (we know that they are both in actual compression by virtue of holding up the truck, I use "relative" to describe the change from "straight-ahead" static loads).
So I gotta go with Brian on this one - I bet the springs are shifting laterally as well as flexing vertically. You are experienceing "stiction" or "slip-and-stick" where the spring withstands the loads until they reach a break pont, where they let go with a "pop". You don't experience this at higher speeds, because the springs are constantly flexing, and the caster change induced stresses are relieved "on-the-fly" before building up to a rapid-release force.
Or not.
Have someone watch trhe front end whilst you cycle full left/right, and see for sure. Let us know, please?
"other peoples junk, is something or other" - Militarypotts 02/07/2011