Hi again Tony,
interesting photo. First of all, this wheel has worn the outer shoulder badly, what does the other wheel look like, worn the outer shoulder too, or the inner shoulder? Because, in the latter case, there is the chance that your axis is just misaligned, i.e. not exactly symmetric and at a right angle with the towbar hitch.
But I fear it could also be the rubber elements, that they are not keeping the wheel vertical all the time under all loads, but have so much soft play that they push the wheel out when you go over a bump. Could be that they are overloaded or just not good enough.
No other help then than replacing the axis incl. the supension elements, with possibly getting a stronger rated axis with the same dimensions.
Tony I am an engineer by training but do not claim to be able to offer faultless expertise over a trailer I have seen on a photo. I can only guess, not guarantee. But these are my 2 best guesses.
Kind regards,
Claus