Why would the paint on a fence that is under trees be bubbly/peeling?

At my university, there are metal fences (simple three beams with posts in between) and the sections located under trees has its paint peeling/bubbling/not shiny. The paint with nothing above it appears to be fine (shiny and smooth). I have walked around all of my campus and this seems to be a trend (didn't do any actual statistical analysis, but will if need be).

I asked a maintenance and all he told me was it is outdoor paint for metal.

The trees are deciduous, I know nothing more specific about them.