Over the years I have seen a handful of wagon wheel failures, various factors ranging from flat tire to encounter with big bump.   I can think of 4 where the front wheel failed on the front wagon with two wagons hooked together, loaded, turning a corner.   A couple of those were axle breaks.   Your wagon looks to be built stout, lots of welded structure under the box, likely not much frame flex loaded.   No rocking bolster between gear and box.   In a perfectly level world all 4 tires would have equal weight on them.   Roads have crown, field driveways seldom level with the road, tires are going to have unequal weight at times.   Then the turning factor puts another dimension of stress on the axle and hub, compounded by pulling a 2nd wagon.

How much air in the tires?   Some wagon builders recommend max 50 psi which I suppose has to do with allowing some tire flex to soak up shock loads.   400 bu load of 60# corn 24000# plus wagon/gear maybe 4000# we have 28000#, are the hubs/spindles rated at 7000#?   Most of the familiar 8 bolt hub/spindle combos handle 5000-6000#, the rims are going to be similiar.