PTO Declined

I’ve been working in IT for the last ~6 years and never had PTO problems. I’ve always booked my holidays first and then submitted them in Workday - well in advance in case of first come first serve basis. Recently heard someone say they had their PTO declined/changed because they’re needed for a release and they normally can’t have PTO during certain times of the year. I know you’re probably meant to ask for time off first before booking something but is this actually a common enough occurrence where PTO doesn’t get approved?