devtrackers.gg
It has nothing to do with being owned by EA. Company departures and resignations are pretty normal in all tech sectors, that's how you secure better pay and benefits for the most part is by moving jobs every few years. It wouldn't be unusual to see someone move from one EA owned studio to another.
Respawn wasn't your regular set of game developers and resignations, especially from the core team, were far from normal up until the last few years; the core design and engineering teams had been together for various lengths going all the way back to the studio '2015', working on Metal of Honor (for EA ironically), Call of Duty through Modern Warfare 2, and both Titanfall and Titanfall 2. I'd encourage you to look at the credits for those games to get an idea of just how much talent was lost in just a few short years.
Everyone had their own reasons for leaving, and the aquisition by EA was certainly a factor for some.
Source: 5 years at IW, 10 years at Respawn