That isn't true. 5 major exctinction events are related to giant volcanic eruptions. 7 are due to sea-level falls, and one is due to an asteroid impact. In all cases there might be a correlation to ocean anoxidation, but I doubt it as a cause.

Eocenic period had high CO2 levels, but it didn't lead to ocean anoxidation. In fact, at the end of the period, the eocene-oligocene exctinction event happened in connection with reduced CO2 and global cooling of the climate. [0]

[0] https://en.wikipedia.org/wiki/Eocene–Oligocene_extinction_ev...