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I’ve never had that happen, but after reading stories of others who managed to pull a socket 478 P4 out of its socket, bending quite a few pins when it stuck to the heatsink, my standard procedure for removing CPU heatsinks is to do it while the system is warm, and to gently rotate the heatsink back and forth to break the bond with the thermal paste instead of pulling straight up.
Silicon is brittle but not exactly weak; I would suspect as you mention that the bulk of the damage had already occurred before you pulled the heatsink off, possibly thermal stress from repeated uneven heating/cooling. Looking at some die photos, there is a noticeable feature line around 1/4 of the way across from one edge, roughly where the straight side of your crack is.