... IB Destroyed vBulletin so Kier and Mike left, they still want a good forum software and they don't believe vBulletin is that any more so considering how they are the real guys behind the original vBulletin Success they decided to build xenForo.
Based on Twitter posts etc I think its been 3-4months.
I guess it was a Joint Decision, but Kier/Mike can clarify that.
There has been a discussion over this and my conclusion was
Xen = Fresh, Balanced, Minimilistic
Foro = Forum (Modern, new way etc)