I've never had such issues with Bittrex, so it may be that you tried to withdraw a big amount of money.
And in such case I find it normal in the world we live in today to require that. It's the AML requirement they have to comply in order to run legally.
I did a quick google search and I have 2 links for you to read.
https://support.bittrex.com/hc/en-us/articles/215282838-Frequently-asked-questions-about-account-verification
https://support.bittrex.com/hc/en-us/articles/231701788