It's not throwing him under the bus. It's distancing him from the investigation. That's both to protect him, in case things favor his story, and to protect the company, if it turns out he was in the wrong.

In both cases, they wouldn't want him having any potential influence on the investigation. To do that, his power within the company needs to be suspended until a conclusion is reached. The parties investigating (likely HR and the other founders) must be able to do their job without fear of retaliation from the accused founder, that would taint the decision.

Because that is what the lynch mob is screaming for and anything else would be labelled as aiding and abetting sexism and discrimination against women by some people?

If I was the person accused, and I was not guilty, I would request to relinquish all responsibility pending investigation to ensure there was no reasonable way anyone could claim I'd interfered with the investigation when it cleared me.

Of course it can also mean the opposite - my point is merely that suspending someone pending investigation says nothing at all about guilt.