Simple. Mueller is squeezing Manafort to get him to turn on Trump. SOP for zealous Federal prosecutors.
But there’s nothing to turn Manafort onto Trump. I’m not sure Trump even knew the guy that well, so what can he know about Trump nobody else knows?
If Manafort is intimidated into giving false but incrminating information where is the profit to Mueller in that? It will blow up in his face big time, even as Manafort goes to prison for the coerced perjury-lying to federal investigators.