Sorry forgot that, my opinion;
Lets start with "he twisted the crank" We've heard the term, a not uncommon event in high performance engines being pushed hard. It's the nature of a piston engine, straight line piston motion is converted to rotating motion through the connecting rods and crankshaft. These are big forces. The battle between strong and light is endless.
Next; plain bearing vs. roller bearing cranks. Both are used on many engines, a difference is plain bearing cranks are forged from one piece of steel and the rods are bolted on. A one piece crank is pretty resistant to twisting 'til design limits are exceeded. Roller bearing (the XS650 uses both ball and roller bearings) cranks are made up of a bunch of pieces, the bearings are installed and then the pieces are pressed together with lots of force.
On the the XS the mains are provided with splines to maintain the correct relationship, the big end pins are smooth press fits, the "press" providing all the resistance to any twisting. If you do a search on "scissoring" you will find this is a "common" issue with pressed cranks, the pieces can shift or twist through time and or hard use. I think time is a factor here, over 30+ years the metal at the press fit can "relax". Steel flows (very slowly) over time. So that's one way the ends can show wobble.
A different problem that pressed cranks can experience is the pressed joints can start to separate, allowing the end sections to move away from the big end pins. Both types of movement create stress on the bearings, increasing wear and vibration and eventually failure.
Pressing apart and reassembling a crank also reduces the the strength of the pressed assemblies, hence welding the joints to prevent movement on rephase motors. For a while big full welds were done, current thinking is more like a few small welds to keep movement from starting, with the pressed joint doing most of the work.
This is not a problem unique to the XS650, Some Harleys are prone to scissoring under hard use. Suzuki had issues with the built up crank GS series also as they kept increasing horsepower. There was a recall, crank welding campaign on some years of GS1100s.
Remember that Yamaha was a 2 stroke company. 2 strokes have to have ball and roller cranks to operate in a fuel supplied lubrication environment. The very first XS's even had roller small end bearings, THAT was an immediate problem, and the roller small end was converted to plain bearing part way though the first years production. When the 447 motor was introduced in 74 they changed the wrist pin from 22 to 20mm to reduce stretching of the rod ends from a "too thin" top section, a left over from the "quick fix" plain bearing conversion.
I have a few welded cranks "in waiting", maybe this year I'll build an engine around one and go flog the crap out of it.