Excel 2019 Mystery Formula

From time to time, a weird formula appears in a cell in an old worksheet without my entering it when I first open its workbook.  When that happens, I cannot enter data anywhere in the worksheet until I delete the formula.  In the past, the formula has triggered a circular reference error message.  Today, a formula appeared but did not generate an error message.  I could not delete the formula or enter data anywhere.  After closing the blank workbook which opened when I first opened Excel to access the old workbook, I was able to enter data in the old worksheet and delete the weird formula.  What is causing this problem?