This could be anything, so making a sweeping generalization is premature IMO.

It could be insider knowledge that Intel/AMD/other has made a huge step forward and has reduced Nvidia's moat (unlikely considering his public support for Nvidia's hardware, but possible).

It could be that the AI bubble is popping.

It could be that he wants to "lock in" the gains that Nvidia has made; having that much capital in one company tied to a pretty volatile sector is risky.

Maybe he thinks that the beneficiaries of AI will be B2B companies like Microsoft with connections to most major businesses.

Maybe it's a changing view on how much compute will be needed to fuel AI developments moving forward.

And this is just the list I could thing of off the top of my head. There are plenty of other reasons this move could be happening. Absent an inside line on Thiel, anything else is speculation.