I have found that FKM has a lower tensile strength than Nitrile (by a factor of 2:1 in my last pull test), and has poorer long term set capabilities. Static Viton o-rings in high pressure applications soon develop the shape of the land, and lose some of the elasticity that helps an oring displace and seal.
I have not noted swelling, but they clearly change shape.
Sudden fracture of both FKM and Nitrile o-rings may be more a function of fillers than the quality of the o-ring compound. Imported o-rings seem to be particularly prone to this, and I see it not infrequently.
While extrusion and o-ring failure due to a loose fitting is indeed a risk, overtightening won't hurt the oring in a hose connection, though it may indeed damage soft brass threads.
But I've seen what you've seen: sudden unexpected failure where the o-ring simply fractures. As suggested above, there may have been too much filler with resulting brittleness, or perhaps a small flex crack that propagated after pressurization due to mishandling of a stiff duro 90 oring during installation, especially if it was installed when cold.
In any case, no big deal. Carry some in your save-a-dive kit and replace it on the spot. O-ring sourcing is a very variable phenomenon. Don't blame yourself.
