Krugerrand
Meow
Disclaimer: I do not know if this is the cause at all, but merely my experience of working on high profile projects...
In my experience on high profile projects, it seems that management can seem to think of the employees as fixed assets that will stay there no matter the level of pressure or hours required. If the project is critical, you need to ensure that the project members get the resources, support, and positive encouragement they need to be successful. If you take them for granted, they may depart at a most inopportune time. I've seen it on projects I've been on, and witnessed it happen to other projects as well.
My speculation is that these people are in high demand, and Tesla is a very demanding company when it comes to their employees. They may need to adopt a more friendly attitude similar to what Apple does, if you've ever talked to anyone who has worked at their campus. That said, I think working for Tesla is a dream job for many, myself included.
Or, Tesla wasn't happy with the progress they did or did not make. Just look around the forum and you'll see several people complaining/unhappy with AP or some other 'computer' issue and how it's been progressing. The departure of these people might in fact be a good thing. It's no secret that working for Tesla/Elon isn't a walk in the park, that's been known for years. If you can't stand the heat, don't walk into the kitchen dressed in a parka thinking you can deal. And it's always 'the end of the world' when some high profile person leaves Tesla. Been watching this happen for the last 5 years+. So far every single person was replaceable and Tesla didn't implode. I yawn at the drama being stirred up whenever somebody leaves Tesla. There are only two people leaving that would truly concern me. I'll give you one guess who they are.