Why Does the System Reset Users to Inactive When I Try to Activate Them?

Your account most likely reached its maximum user count.

Each account has a defined number of permitted active users. You can fix this by doing any of the following options:
Note: A deactivated user can be reactivated at a later date. However, a deleted user will be removed from all reviews, projects, and deliverables on the account. Any assigned work will be reassigned to the account owner.

Once enough user licenses are available for the number of users you are trying to activate, you can activate inactive users and create new users using the activity status that you choose. To check your account usage, see Check Your Account Usage.