There are two important effects that contribute to the pyramidalization of CF3 radical:
1) Electron-electron repulsion;
The unpaired electron on carbon repulses more with the lone pairs on the fluorine atoms in the planar geometry (predicted by sp2 hybridization) than in the pyramidal geometry (sp3).
There is also a hyperconjugation-type stabilization in the pyramidal geometry, between the orbital that contains the lone pair and the C-F antibonding orbital.
In the planar geometry the orbitals are perpendicular and no overlap occurs.