The rims will also crack when they are just worn out by the rubbing of the brakes. I think it is unlikely they cracked just because of the added weight.
Rims generally crack in one of two places:
First is just what you reported, the brake track cracks from abrasive wear. I've had several rims fail that way in from 11,000 to 30,000 miles depending oh how they were used, how much bad weather they were exposed to and how durable the rim was to begin with. Very light rims are, obviously, less durable and less wear resistant.
The second mode of cracking is around the spoke holes which is a fatigue failure or caused by excessive spoke tension or poor rim design/material.
The tongue weight of a trailer is unlikely to accelerate either mode of failure.