Think I "broke" the game

Morphis

Was testing a deck with kiora against story mode, breaker of armies boss(BFZ boss befor the last one).

Ok usual stuff happening.
He summons an Ulamog.
Nice! That's MY Ulamog... Exert influence.
The very next turn he kills it.

Ok whatever...
Then I keep controlling... The game lasts long...

Until Ulamog comes back...
Not from him, from my deck!

I DREW ULAMOG FROM MY DECK. There is no Ulamog in my deck of course!

My guess is that after a certain number of decks are generated(and believe me I drew MANY CARDS) the game reshuffles the graveyard in the deck.
It was really unexpected XD