I doubt mowing sideways has much to do with this.
If the left and right side axle shafts are the same part number (and will interchange) I would be more inclined to suspect that a poorly heat treated batch of axle shafts found their way into the assembly line that day/week/month.

But, this kind of fracture/failure can be caused by seriously overloading the axle with weight.

How steep a slope? How much do you weigh?

You are doing something that is putting an excessive load on the 3/4" axles.

Give TT a call and talk to the man.

I'm 222 and my dad who is doing the mowing this year is a bit under 300.
Side hill wasn't too steep, the downhill one was pretty steep.

This winter I plowed several times and did some tire spinning. My dad has been primary mower for last 12 months.

Forget "tire spinning". Any time the tire is spinning, it has been relieved of most of the torque loading and that lessens the chance of a "twist out" failure.
My bets are still on a batch of poorly heat treated axle shafts, and that possibly limits the failure prone units to a few in a very narrow time span of production.
Something that only a tally of failures versus serial numbers/production dates could verify.

I was told by an engineer tire spinning then catching was a big stress on the axle. I dunno.

I talked to Derrick again who said mowing sideways and our weights shouldn't be an issue. Said it was odd and hooked me up with another axle and oil seal. Not looking forward to this again.

It's like tiling a floor, cool to do once, but a chore the second time.

Tire spinning.........and then catching SUDDENLY (like going from zero traction to 100% traction) could produce some twisting (torsion) in the axle shafts, but I would not have thought that prevailing traction afforded during "snow plow" conditions/surfaces would be great enough to induce enough torsion to damage things this way.
Unless YOUR snow plowing technique is radically different than the rest of the plowing society, I would have figured the instance of snapping axle shafts (or other failures) would be well known and documented.

If the OP snaps either of the new production axles that will tell us a lot.

eBay has k46 axles made better.

The photo of the broken axle indicates flexing damage, that accumulated over time. About 50% of the axle cross-section is oxidized black, so that would be damage that is more than a month old. The fresh break is the lighter metal in the center(more or less) of the shaft. So the axle was already damaged when it finally snapped.

agree with Eric's observation. In fact - if that machine has low hours - would take issue with tranny mfgr on defective axle shafts from gitgo...

Sure would be worth a toll free phone call to Tuff Torq... 866-572-3441