One would think that by now these ejection seats would be designed as failsafe.
In a DEI world?........................
They are. They are safed on the ground with pins the pilot removes during preflight prep (using a checklist). Most incidents like this occur when something snags the seat ejection handle.
Another scenario is that the pilot wasn't strapped in and accidently activated the handle. The parachute is integrated into the seat and the pilots straps it on upon entry and flight prep. He goes with the seat but is separated after ejection.
Incidents like this have happened before.
The other possibility is seat or chute failure to achieve safe deployment. We just won't know until the accident investigation.