The way I understand it, which is vaguely, Massie already lost the primary or something so he can't run in the regular election. But he could run in a special election, which would divide the republican base and help a democrat to win.

It is convoluted though, and beyond my limited comprehension of political complexities. Republicans tend to be very underhanded like this though, so republican-controlled states have convoluted laws which enable them to do these kinds of shenanigans.