My Billing Account's Tier was again downgraded from Tier 3 to Tier 1

Refer:
My Billing ID - 015818-71F10D-5479.., Project Number - 6026541999..
My Billing ID - 01D5EF-5DBBF9-8030.., Project Number - 6498477305..

Why is the billing tier downgraded without warning or reason!! This is extremely hazardous to our business as we run a 24/7 Support voice agent. This is not okay. Need resolution quickly.