Typically due to vibration... Ive seen the "coating" on these frames allow water/rust occasionally to occur. This with vibration could be the reason. Probably nothing you specifically did.
It just needs to be welded, maybe add a few supports.
(If you go to a legit fab or race car shop, It will LAST and you will like the way it looks & performs. If you go down to your local ""I can weld your trailer up for you with my 900AMP ARC welder"" , the results will 'vary'... LOL
Pictures are for reference.. Some simple MIG, if your able, find a good TIG (TIG is not necessary, just cleaner and could get more strength in a smaller space)
View attachment 92286
View attachment 92287