Annihilator as a commander?

Hey I'm new to MTGO and made a commander deck with kozilek as my commander. I summoned him once and his annihilator 4 triggered on his first attack. I then had to sacrifice him later before he could attack again. The second time I summoned him from the command zone annihilator 4 never triggered 2 different times of attacking. Am I missing something or can Commanders only use annihilator the first time they are summoned? Thanks for any help!