If the tires are cupping (that means treadblocks are worn in an on-off
pattern- one up, one down) it is likely due to the tires hopping- same
as the wear from bad shocks or struts.   Since most trailers don't
have shocks, that's probably what's going on- especially since you've
been carrying a lighter load.  Often, bias ply trailer tires which are
load rated will solve the problem.
Hope this helps.

http://tinyurl.com/34w29g

One of the folding 4' x8' kit trailers.   It had several
trips from S. Calif to Salt Lake City loaded to the gills
without this kind of wear.

Recently, been towing a honda CBR1000, about 475lbs.  3-4
thousand miles.  Major wear on tires.  ONLY change was
installation of oil filled hubs.

??????

thanks

gary