All suspension bearings were serviceable, definitely not enough slack for the spring/shock to be hitting the crankcase.
And I agree the crack clearly does show some signs of having been there at the bottom for some time as it is discoloured, but what could have caused it to crack and subsequently fail.
I've repaired a couple of XR400's with corrosion issues which cause this but as can be seen here there is very little corrosion in this instance.