Ahhh the joys of aluminium...
Unlike steel, aluminium has no elastic limit. This means that once you start flexing an aluminium profile, it starts to fatigue....even if the degree of the flexing is only be a minute amount the structure of the aluminium will be changing, and weakening.
Steel on the other hand can be bent back and forward an infinite number of times without detremental affect, so long as you dont bend the steel to the point of its elastic limit. ( pretty much the point where the steel stay's in its new 'bent' shape and doesnt spring back) then you will be fine.
In this case you never know what the trailer was carrying before this particular boat or even how it was loaded up in this example. If the drawbar was overloaded and subjected to a cyclic 'bouncing' effect then its inevitable that something was gonna give....at some point or another.
Aluminium can be an 'awkward' engineering material due to this phenomena and requires extra thought when used in the design of structures.
Now dont even get me started on those high speed aluminium cross channel catamaran's