Well, that depends on the plane and the circumstances. The most likely candidates are either being out of trim (in the roll axis), not enough airspeed before taking off or both!
If the roll trim is out then the plane can start rolling after take-off until you have a chance to trim it out. Usually it is a slow to medium roll but if the trim issue is bad enough it can be severe.
Not enough airspeed before lifting off is another likely cause as this can result in:
a) Not enough control authority in the ailerons to counter motor torque (front mounted/tractor motors most often roll planes leftwards from the torque they impart on the airframe)
b) Wing stall. One wing can stall before the other resulting in a roll due to aerodynamic assymetry. Or the wings can stall at full throttle and the full throttle motor torque simply then twist the airframe over.
A couple of my warbirds exhibit these traits. My Parkzone Mustang has a noticeable torque roll on hand launch to the left that needs a firm hand and for throttle to be backed off to about 80%. I watched one of our group launch his own for the first time at full throttle and it corkscrewed on him, caught him by surprise and augered in.
My larger Spitfire also displays all sorts of shenanigans on take-off from its wheels, and the motor is so powerful and tailfin ineffectual at slow speed the thing will often try to roll severely on me and jink left if I lift it off early (forced to do often from less than optimum "runways"). The darn thing has twice done a complete 180 turn on me on take-off and almost crashed.