Like I said, inexperience or over-ambition. Either a new DM who just wanted to use a cool-sounding monster and didn’t realize it was way stronger than they thought it would be, or a DM who tried to homebrew something and accidentally made it way harder than they meant to - which could itself either have been the result of being new to home brewing monsters and not having a good feel for it yet, or of trying something
really wild that even with your experience you couldn’t accurately predict.
EDIT: Or as
@prabe points out, a DM who mis-evaluated third party material, which could come either from inexperience with evaluating and vetting third party material, or from said third party material being so out there the experienced DM didn’t know how to evaluate it but wanted to try it anyway.