Telegram wants to introduce a feature where they request access to your address book, request access to your photo library, search your photo library using CoreML for photos that appear to be indecent, and start sending them to all of your contacts. Telegram is really adamant that this is an excellent feature and they refuse to remove it when Apple denies them over it.

Or probably something else. But you get the idea. Telegram could have tried to introduce behavior that violated Apple rules. Or they could have snuck existing rules violations past Apple in the past and Apple finally noticed.