You may need to adjust the tongue weight at the front of the trailer. You should try and shoot for 10-15% of the total load without overloading the capacity of the coupler. This could also be an issue with the shocks on your tow vehicle. I've seen nothing but problems with the vehicle tow packages that use air-bags when you are using the standard UFP surge brake system.

The first thing that I would try and do is see if you can recreate the problem while using a different tow vehicle, preferably one that does not have any kind of equalized hitch or tow package. You want the trailer to be level with the ground.

You may also have a rotor that has been warped. If you replaced the calipers recently, you may have unknowingly had a damaged rotor as well.