I go by my past experience with companies I've worked for. The only times I have ever seen two or more senior staff members leaving at the same time is either due to:
A) An unhappy work environment finally forces a senior staff to resign, and other staff follows in support.
B) A senior staff has plans to start his own company and poaches other staff members to join his new company.
C) Staff is being fired, let go or forced to resign due to serious misconduct such as industrial espionage or sexual harrassment.
Now I have no idea what is happening at Cryptic, but I heard Heretic is joining another game company so that reduces the possibility of (B) being the cause, leaving (A) or (C) as more plausible reasons. Either way, its never good news when multiple senior staff members leaves a team, especially when Gozer was just given a big assignment to revamp PvP a month ago.