Is it normal that Mass Effect 3 automatically opens Priority: Cerberus Headquarter after the ending credits scene?
Solution 1:
Not normal.
After loading my game, finishing, and letting credits roll I was returned to main menu.
Considering you've done nothing that would prompt the game to loop you back to a save-state after credits, it appears you've found an odd glitch.
Solution 2:
Playing on PS3, I've always been returned to the Normandy before the assault on Cronos Station, after finishing ME3. Never to the title screen. That's the point of No Return™ so it makes sense. It lets you go back and play DLCs, etc., without having to find an old save you may not even have.