They were likely pissed because they were afraid it would make them look bad to management - "How come no one on the backend team could find time to build this,

They were likely pissed because they were afraid it would make them look bad to management - "How come no one on the backend team could find time to build this, but one front-end guy managed to despite his regular workload?" It's an easy conclusion to get to, whether justified or not. No one likes being made to look incompetent, regardless of whether they actually are. I have the impression you didn't ask for permission before you did it - the dynamic is wildly different if you start things off by asking if they'd be okay with you taking some time to build it, then involve them in your process as much or as little as they want. If that's what you actually did, then my analysis is way off.