It was already fermenting when you hit it with your O2, it just hadn't starting showing the more easily visible signs at the time. How was the slurry treated? You says 4 weeks old...was it washed yeast from a prior batch? Did you pitch it into a starter first? How was it stored? It sounds like you used some old-ish, dormant yeast that just took a while to get going. In the future, give yeast that age a starter to wake it up and strengthen it before pitching it into a beer.