The primary causes of turbulence are convective currents, wind flow obstructions, and wind shear.
Convective currents are strongest on warm summer days, as warm air raises. The area right below cumulus clouds will often be very turbulent.
Obstructions to wind can cause mechanical turbulence, such as buildings, mountains, etc. When flying near such obstructions, a helicopter pilot must be very vigilant.
As there is a significant change in winds speed and/or direction, there is often turbulence at the boundary of wind shear.
FAA AC 00-6B Aviation Weather pg. 17-1