After you remove the FEL but before you disconnect the hoses, wiggle the loader control lever all over to 'relax' the hydraulics and let the FEL settle onto its support legs & buckets.
Basically, what I figure happens is the FEL is in a position where it is being supported by the hydraulics, not the legs. As the FEL settles over time, it pushes on the hydraulics, presurizing them.