If I had to guess it would one of two things.

Having the jacks down flexes the trailer frame.
Perhaps this causes the lift supports to bind.

Or maybe the jacks block part of the lift system
when in the down position.

Personally I like to get the jacks up as soon as I
do not need to enter the camper. This helps to
prevent me from driving away with them down.

In article
<7dc9cb88-ef37-455a-9320-cf672e1f1e27@i7g2000prf.googlegroups.com>,

Leveling jacks, even stabilizers (not really "jacks", per se), can distort the
frame of a trailer if they are used aggressively.

I have done this with improper adjustment of the "scissors-type" leveling
jacks on my travel trailer.  In this case, the screen door will hang-up on the
threshold.  Once the jacks have been retracted, all is well and the door
operates without binding.

The same concern applies to the lifting mechanism on a pop-up camper.  If the
stabilizers are tightened aggressively, but not properly or equally, the frame
of the camper may be distorted enough that the lifting mechanism does not
operate smoothly or properly.

My (bought new) 1987 Starcraft Galaxy's manual specified the same thing:  Do
not raise or lower the roof with the stabilizers deployed.

Given the raising and lowering of the roof, as well as the sliding in-and-out
of the bunk ends, it is probably a good practice to deploy or stow these items
(that depend on rather close tolerances to work properly) WITHOUT the jacks
down to ensure the best possible performance.

On Mar 9, 7:36 pm, chrisc...@hotmail.com wrote:

All manufactours say this because you may have twisted the frame
enough to do some kind of damage to the lift system, I had a coleman
in the past and it said it tin the manual, and my dealer explained it
during the predelivery. So it is always a good idea to be safe and not
sorry, this rule also should be done by larger travel and 5th wheel
trailers as they also can twist the rig around enough the cause
troubles.

Happy Camping