If a fungus had a photosynthetic symbiont, it would have a good reason to compete for height. In fact, an algal symbiont is a leading theory, based on chemical traces that point to algae in the fossils.
Interestingly, lichens do this today: they are composite organisms of fungus and photosyth (algae or cyanobacteria). They don't currently get very high, but perhaps that's just because plants are so much better at height that all tall forms have gone extinct.
If there's no photosynthesis going on, it's harder to explain. Perhaps it's handy in a very low-growing world to lift most of your energy-storage mass or hard-won water above the ground, away from predators?
It's also not uncommon to find fungi that are several feet long growing on trees and such. Not sure if the article specifically said mushroom or if it was just a fungi they found a fossil of.