Maybe the grooves were not formed while Phobos was/is a moon.
Perhaps this was a rolling rock on the surface of Mars. Grooves formed from the rolling. A nearby very large impact spewed debris, including Phobos, off the planet. Phobos entered a stable orbit.
Utterly preposterous!
You must bear in mind that these celestial bodies are extremely fragile, low-density conglomerations of loosely-packed boulders and dust (possibly also ice), with a covering of regolith, held together by extremely weak gravitational forces. Think "7-layered wedding cake" or slightly denser "dust-bunny."
They are simply too fragile to have survived such a hypothetical cosmic collision intact.
Regards,