Most likely you have some monitoring tools running which currenty use your old root PW. Depending on you defaults the account is looked after 10 tries for 900sec. or so.
You can reboot the host and if you hurry you can manage to try login trough SSH and investigate from which IP the faulty login comes.
About vCenter. It normaly doesnt use the root account of the ESXi. Only during the joining/onboarding process the root account is needed. As soon the VPXA is installed also a vpxa user will created on the ESXi and together with the certs/keys the vCenter use this one instead of root.
Regards
Joerg