There's a fair chance this has nothing to do with 51% and Peter Todd just felt like cashing out some coins and wanted a good sounding reason.
What the heck, why would he want a reason? Is there some sort of obligation for bitcoin core devs to keep their money in bitcoins? Do they have to publicly explain when they cash out? I don't think so, it's their money they are free to do how they like.
Just human nature. People often feel the need to explain their actions.
Can't be just human nature. He knew he would be much criticized for this act and he still did it, though he could do it without anyone noticing. No, he truely started doubting Bitcoin, that's what it is. He had a plan set up in advance and he executed it, because he believes in the need for decentralization of Bitcoin more than many people who criticize him.
That's true it takes a set of bravery and willpower to execute a strategy even when people say your foolish for doing so.
The problems with centralized mining due to the development of ASIC's and the slowly decreasing amount of full nodes are issues that Bitcoin will need to face in future and find solutions to.
That said explaining your actions and rationale even if people don't agree is still a good thing to know.