A qoute from Rimex.com might help answer your question:
"The introduction of the radial tire has had an important and serious impact on rim performance. Due to the special construction of the radial tire, it has a more flexible sidewall, causing more movement in the bead area.
This deflection and movement in the bead area, which causes flange walking and rocking, and the added stresses of higher operating pressures, are the major reasons for rim back section failures and flange failures. The radial tire transfers over 75% more stress than the bias tires to the rim base."