Check this post Twitter’s Lead Architect Leaves: It’s Not You, It’s Me from Mashable!:
"While I’ll give Cook the benefit of the doubt, I can’t think of a more pressure packed job than being responsible for the scaling of Twitter, a service that has continuously struggled to keep up with its growing user base. Was Cook forced out as the scape goat for Twitter’s technical woes? Who knows, but as someone that was with Twitter since the beginning and likely an owner of some valuable stock options in the redhot-in-spite-of-itself startup, it seems unlikely he’d leave completely on his own terms."