Axel risers

And yes  risers for sure increase the stress on the frame at that attachment point. That's because they create a longer lever arm trying to bend the attachment point backwards and upward when you hit a curb or your tire hits the front edge of a pothole. This rotational force (called a bending moment) combines with the moment created by the torsion axle rotating back and up, and the g forces from the rest of the trailer being pushed up,  to place severe stress right on the rear of the riser mount where it's attached to the frame. That's were your crack is I'm guessing. 

BTW leaf spring axles put a lot less stress on the frame in a pothole or curb impact event than a torsion axle does, so getting a trailer with leaf springs is another way to try to avoid this kind of problem. 

Also, don't put heavy stuff in storage compartments located behind the axle, don't load bikes or other gear I  the rear bumper.  All that stuff back there is the worst thing you can do when loading a trailer with a torsion axle. It also makes the trailer more prone to sway. Put all your heavy stuff in the tow vehicle or leave it at home. If you absolutely  have to put it in the trailer put it as far forward as possible. 

Scubasteve,

I had a set of 3" risers made and installed a set on my 192.  Made a nicer towing trailer by doing this and either end of the trailer does not hit bottom any more.  Lippert does say that the warranty is voided when spacers are added between the axles and the trailer.  The insurance company asked about modding the suspension also. I used grade 8 bolts,nuts and washers and torqued them to 150Ft/lbs.  I have also checked the torque a couple of times since installation. If you crunch some geometry numbers on the risers you will find initial the shock of hitting a pot hole is absorbed by the tire and the swing-arm increases torque in an ark.  Applying the brakes sharply and for a length of time transfers more torque to the frame than a pot hole. Next time you apply the brakes look at the trailer rotating on the center line of the axles.  As for cracks,I have not seen any.  The spacers I had made were 9" long and the width of the frame. The bolt holes aligned with the existing holes in the frame and axles.  I think I have a drawing of them if you are interested for comparison.

S