My thought is that  the front portion of your brackets and/or their welds were holding them to the frame but the rear welds/bracket areas had failed, so that they basically peeled off when you went back over that curb.  The risers do increase the loading on the brackets and their frame attachments in that scenario, but not to the extent that something like this should happen. 

Check the other posts, It seems the strap bracket from the frame is used to tie the camper to the axle. My black tank strap is the same strap you are talking about , but on the axles. My strap broke on the way out to colorado on a maiden trip dropping the black tank pipe on the highway, breaking the gate valves. I was lucky not to lose the black tank pipe. I was able to use a coat hanger to tie up the pipe, but I could not use the black tanke and FR would not support the problem. So first trip out, no bathroom ;( also the grey tank pipe was damaged when the black pipe fell dragging o nthe highway for an unknow amount of time.  Check out the other posts and someone suggested filing a complaint here: I did just order some fluidfilm on amazon to undercoat my frame and inspect, I will post pictures today. I highly suggest we all use something like this annually!!

Also, if you have not done so, file a report with the NHTSA about this. That also might get some attention.

https://www-odi.nhtsa.dot.gov/VehicleComplaint/ - EDIT BY MODERATOR. The axle isn't held to the frame by a "strap" it's held by 4 large bolts. Many owners here have added lift kits and can for sure tell you this, as can I.

You are right on the bolts. I crawled in their today to check mine out. From the original photos on the post, it looked like a strap was in between the top beam and the axle beam which then corroded. My welds appear ok, not the best welds on the inside under the camper, but they look ok. I used rustoleum rust converter to remove the rust and primed them. Tomorrow they get fluid film rust prevention I heard others talk about.  I will check the welds yearly.