Any idea what causes this?

In no particular order

Improper tune, timing too slow Do NOT necessarily time car "by the sticker" which was for best smog, not best driveability

Generally, condition of ignition components, cap, rotor, wires, plugs

vacuum leaks, including bad brake booster and or hoses and fittings

problems with carb and especially idle circuit or even just incorrect idle mixture

generally poor engine condition, worn rings, valves

worn timing chain/ cam drive

In rare cases, problems with transmission, IE converter