To avoid confusion over debits and credits, avoid thinking of them in the way that they are used in everyday language, which often refers to a credit as increasing an account and a debit as decreasing an account. For example, if our bank credits our checking account, money is added to it and the balance increases.
One way to reduce the frequency of this occurrence is to maintain an air pocket at the top of the water heater tank. This air pocket will form in the tank by design - however, it will be reduced over time by the everyday use of your water heater. To replenish this air pocket: Turn off the water heater.
It could be from a number of reasons. Normally, hot water pressure out is determined by the pressure of the cold water going in, of course. However, excess floating scale and debris from the inside of the water heater could have clogged the various hot water sides of faucets, washing machine mixing valve, etc. Usually, you can hear this debris bubbling when the heater is on.