Griffis put it bluntly: "Yes, I will be leaving if a few weeks for a new opportunity. That's pretty much all of it. It has nothing to do with PvP, STO, or Cryptic. It's just time for me to do something different. I had planned on telling everyone on the forums at some point, but probably closer to when I was actually leaving (I'll be here for a few more weeks)."
He says that PvP will be transferred to another developer, and he's not going to talk about what's in store for that part of STO. Griffis previously left Cryptic in 2010 but returned shortly thereafter.
[Thanks to Don for the tip!]