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Is it the force of recoil that causes cracks in the frame where the water table meets the standing breech? I wouldn't think chamber pressure would be involved but I'm no physicist. Thanks.

The point at which the action bar meets the standing breech is the point of maximum stress under firing. The action bar must resist a bending moment because of forces acting upon the standing breech and the hinge pin (and the draw, if it's fitted to touch the circle). In an O/U this bending moment does not exist because the action walls act as a shear web.

Here is an FEA analysis that Boxall & Edmiston did:

This was a problem with some early breech loaders where the angle was at 90 degrees. Cracking occurred either at Proof or after prolonged use. It was soon understood that it could be cured by a radius at the point between the breech face and the action flats. I have a very early hammer gun with a sharp cut off by Turner of Reading. It's not shown any sign of failure yet although I very seldom use it. I would never think of trying to submit it to Nitro Proof. Firing the gun produces a downwards flip of the barrels which puts strain at that point and the reason some guns have top bolts. Lagopus.....

http://www.boxallandedmiston.co.uk/shotgun-anatomy/stress-analysis-in-gunmaking/

100,000,000 newtons/sq. meter = 14,504 psi yield (not ultimate) strength.
Boxall & Edmiston stated in 2011 that the frame was EN series chrome moly steel

Shotshell pressure is distributed in every direction; most, likely down the (unobstructed) barrel, some contained by the chamber walls, and some directed toward the standing breech.
Recoil is directed back against the standing breech, up (depending on the axis of the bore), and (in a SxS) laterally and (maybe) down initially.

Peter Blakeley "Successful Shotgunning"

This might spare some verbiage...but not likely Possibly we could restrict our comments to shotgun design, metallurgy and mechanical engineering.
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=609802

A simple experiment to prove this flexing action is to stick a piece of paper tightly across the top of the breech and fire the gun. You should find that the paper is torn. Nothing to worry about as this is quite normal and everything flexes back again. This is one of the places that guns crack open undergoing Proof firing. Very rare but the extra pressure will show this weakness. I have only had one gun fail this way which was a double 20 bore black powder proof gun undergoing nitro proof. The action gaped quite a bit but was skilfully welded up and re-submitted; this time passing o.k. Lagopus.....