While I agree making a bad loan is the most likely reason for a bank to lose deposits, I wouldn't go so far as to call it "the" reason.

Robbery, embezzlement, or natural disaster [1] can also result in loss of funds that would be covered by FDIC, and those risks exist for cryptocurrency too, albeit in somewhat different forms.

[1]: see this paper, which found that "disaster damages play a significant role in bank failures": https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2506710