The rod side of the piston, initially the low pressure side, becomes the high pressure side if fluid is getting past the piston seals, as I'm sure you know. I'm wondering if, in some cases, this high pressure then starts bleeding through the valve since it's only metal on metal between the spool and valve body.
Anything is possible, including a leaking hose.
But most of the time, I'll bet it's the internal seals in the cylinder.