The metal is warped by having the sand beat on the surface causing the surface to expand. The whole idea with sandblasting is to limit the energy of the stuff hitting the surface such that it will remove the bad stuff and not damage the metal.
There are two ways to put too much energy out.
First is with too much pressure. If you have too much pressure every piece of sand will cause damage. I run my pressure blaster at 40PSI.
Second is lower pressure with too much sand. The more sand in the stream the more energy you have to beat the surface until it warps. I turn my pressure blaster sand valve off and just crack it until I see sand flowing in the airstream.
I have never warped a panel with the above settings. I have warped metal on purpose experimenting.
Why you ask this makes a difference in the type of blaster...
A siphon blaster does not generally have a means to regulate the flow of sand. It is possible to warp a panel at 40 lbs because the siphon blaster will allow too much sand in to the stream. It is also not as efficient at using the media.
I hope that clears up the picture.