You should look at the Vault injector logs. There’s probably a deployment in your cluster called {{ template "vault.fullname" . }}-agent-injector assuming you installed it via Helm.
You may also check the Vault namespace for API events.
It’s not uncommon for the injector pod(s) to be evicted when the cluster is under pressure.
Make sure you have enough replicas running to avoid unavailability.
Try to spread the pods adequately using topology spread constraints and use priority class.