typically bad ram packing would be the cause (on older units)
or
a faulty safety valve in one of the cylinder rams(typically on newer units with fluid displacement type systems, rams with no packing)

rare but possibly a bad spool in the control valve

can you be more descriptive as to what type of mast this truck has and the symptoms?
2 or 3 cylinder setup? (duplex v/s triplex mast)

is it the forks only when in freelift without mast channels extended?
or does it do this when mast is up high with channels extended?

if it does this with just forks and carriage only then it is most likely a safety valve in the center cylinder.
If it does this when the channels are extended and the forks drift all the way down and then the channels start to fall too then it is probably a safety valve in one of the outer cylinders