Odd tire wear

The biggest reason for one tire to wear on the outside is the driver "pinches" the bottom of it against a curb in a turn. Usually that typically bends the spindle on the FRONT tire...when moving forward. And that is the tire you describe.

A good large trailer alignment shop can straighten it. Plan on $400 for the alignment and fixing the issue.

You might want to look closely at the other tires, to check them as well. If the tire on the other end of that same axle is wearing on the inside, that indicates other possibilities. Like out of perpendicularity.

Oops, JIMNLIN is correct about the over - under inflation causitives!

WoodGlue

In looking at the tread it looks like the outer edge of the tire was worn more than the rest of the tire, and maybe that was why it had failed?
Any ideas as to why the tire would wear like that? Could it be a bent axle?

Did the tire actually fail or just a odd wear pattern ??

Bent axle or just the axle stub most likely if the tire is worn on one side. Take the trailer to a big rig trailer repair shop . They can easily determine the cause and bend the axle to specs.

Or if its a ST tire then its very possible the tire had internal "tread" belt separation on the out side of the tread which will cause a wear pattern such as you mentioned.

To much air pressure causes the tires tread to wear the centers

Under inflation will wear the inside and outside of the tread to wear more.

Bent axle. The PO probably jumped a curb or hit a serious pot hole.

Yes

It could be anything from under-inflation to over-inflation or even, as you mentioned, a bent axle. The outer part would probably indicate over-inflation of that tire.

WoodGlue