Maybe they freaked after the Cantor rout. they know what it means better than even cantor does, better than Brat does, and they couldn’t risk the morale boost to the conservatives, plus they knew they could pull off a Mississippi win.
Compare the headlines:
McDaniel gets the big nod form the republican base/voters
VS:
Cochran ...well, you know
Let us not forget they are now defunding Brat out there.