I'm guessing that area had some bad material. maybe rotted, who knows. Ivory should not do that. I agree it can do strange things and does absorb moisture and should be periodically oiled and it can dry and crack but its been used for ages for all kinds of stuff.

remember also that (I'm assuming its not elephant ivory) the mammoth stuff is different. It has been buried in the ground for thousands of years and has begun the fossilization process so depending on the minerals absorbed and the condition of the stuff it could have weak areas.