I have 3 thickness planers, never had a kickback ...
Taking too big of a cut should just stall the motor.
The anti-kickback pawls are not working.
The feed roller is dirty and starts and stop during a pass.
The tables are not in the same plane on either side of the machine housing.
A table on one side is loose.
The knives are dull or misaligned, one set being too high.
The belt is slipping.
The wood has hard and soft spots like knots and such ....
I have no clue.......
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)