What causes an engine to 'buck' at low speeds?

[DonC]

What you're describing could be either the state of tune of the engine as indicated above or a syndrome of driver induced oscillation similar to PIO (pilot induced oscillation) when flying.
More common in rotary wing aircraft than in those other things where the wings don't move. Sort of like a short coupled aircraft where the distance between the wing and the Hstab is short and small movements in pitch control have large results in pitch attitude change.
Probably more common in the big block cars than in the small blocks because of the torque put out by the BB's. Caused by your body moving just slightly fore and aft as a result of the car's acceleration and that movement being transmitted down your leg to the throttle.
Like most other things with our toys it can me moderated by concentrating on being smooth with throttle modulations at slow speeds. In some cases it could also be addressed by increasing the throw of the throttle linkage so very small movements of your foot on the pedal would have less effect on how much the throttle is opened.
DonC