Definitely caused by rust - I would replace both LCAs as a preventive measure. Be thankful you weren't in a W210 with suspect spring perches! Interestingly, it seems that most of these major catastrophic failures appear at low speeds. Probably due to greater torque on these components when working against friction between the tire and the road.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280