Few reasons:

1) Non-performance: If a founder takes investor money and then fails to deliver on the KPIs quarter after quarter, that would be a legitimate reason for getting fired.

2) Politics and Personality: Often founders accept to bring in a CEO, and then there ensues personality clashes, power struggles, and related politics. I experienced this myself, and have written about it elsewhere. [If I Were 22: I Was Fired from My Own Company]

3) Disagreement on Strategy: Founders and Boards may not agree on strategy, and that could result in a leadership switch.