This is actually pretty consistent across companies: being the most senior person around when the last manager leaves. Sometimes it's a senior dev, sometimes a PM, heck I've seen it be somebody from the UX org. The path of least resistance is a quick promotion from within. As many of us have experienced this leaves us with a non-zero amount of managers who are both unqualified and uninterested in their role, but accept it for status (you're supposed to get promotions after X years right?) and pay reasons.