Omnipeek defines a sever broadcast store as "a sustained level of 'all stations broadcast' packets (the destination physical address consists of all 1s) has met or exceeded the threshold." The default threshold is 500 packets/sec with a sampling period of 3 seconds. This can be modified by right-clicking on a packet and selecting Expert EventFinder Settings. Expand Data Link, and you'll see Severe Broadcast Storm there.
In your controller, are your VAPs configured to "drop broadcast and unknown multicast" and "convert broadcast ARP requests to unicast"?