As far as whether this might be the cause, one possible scenario: the board hired a law firm to investigate, Sam made statements that were contradicted by credible evidence, and that was the fireable event. Brockman could have helped cover this up. Again, not saying that this is what happened but it's plausible.
BTW Rubin's $90M payout a) caused a shitstorm at Google b) was determined in part by David Drummond, later fired in part due to sexual misconduct. I would not use this as a representative example, especially since Google now has a policy against such payouts: https://www.cbsnews.com/news/andy-rubin-google-settlement-se...
People who said Google should have withheld Rubin's compensation are operating under the assumption that Google would have prevailed in the inevitable lawsuit.
But since you brought it up, the fact that Google changed their policies in response to the Rubin (and Drummond) situations and did not caveat their policy with "except in the case where there's a performance bonus, which we'll still totally pay out" implies that it was a choice to begin with.
Also, even if there was a performance bonus that Google felt they might be forced to pay in litigation they could still have fought it to demonstrate a commitment to not rewarding executives preying on subordinates and to preemptively address potential employee backlash, which was entirely predictable. Google has effectively infinitely deep pockets and did not need to preemptively surrender.
And in addition, Drummond and Brin were both involved in the decision and both had affairs with subordinate employees. So, while I wouldn't say that Google had an active goal of "reward abusers", it's quite plausible that the specific, small group of men making this decision on Google's behalf may not have been particularly inclined to punish behavior arguably similar to their own.
Again, you're tackling this from the frame of mind of being certain that Google would win. It's not about the money: $90 million is almost certainly cheaper than what this case would have cost. It's about the reputational damage: Rubin potentially winning a settlement against Google would have been immensely embarrassing.
It's all about doing what's in the best interest of the alleged victim. She would have probably had to testify at trial. And imagine the hell it would have been to have a settlement paid out to your alleged abuser, thereby implying that you're a false accuser. Juries can be unpredictable, its easy to see why Google decided to find acceptable terma to part with Rubin.