WAG that the storage rack was not properly connected with substantial anchor bolts and maybe no footing to retain anchorage.
Storm surge and tidal action worked debris through the garage doors and acted as battering ram on the uprights. There is no sheer designed into the straight legs and with the weight of two cars up high repeated back and forth tidal action may have worked it to failure.
It would be interesting to know how high the surge was there and to have a look at the steel members, the fasteners, the garage doors (were they simple plywood to enclose a balloon foundation to locate home above flood level) and also the attachment to the slab (that appears to still be in place).
It does suxx, I've had to leave our home on several instances with sprinklers running as wildfires approached. Fortunately left no vehicles behind and never had a loss.