Relax your front foot
Mechanism / Why It Happens
The more you try to spin the board, the more tension builds in your leg, causing you to kick the board forward.
In a proper kickflip, popping the tail lifts the nose, which then pushes your front foot back. From that position, you can apply a diagonally downward force to the board, keeping it under your body while making it rotate. However, if your focus on spinning the board is too strong, your front leg becomes stiff immediately after the pop, preventing the nose from pushing your foot back. If you try to flick from this state, your force is directed too much toward the nose, causing the board to shoot forward.

What to Do
Use your body's natural reflex to force your leg to relax
If your foot becomes tense because you're trying too hard to flick, simply telling yourself to relax