Bomba & plena in a Puerto Rican supermarket

On the first day of our Dec. 2011 vacation in Puerto Rico, we stopped for groceries in Ralph's Food Warehouse, a US-style supermarket in Humacao. We were amazed to find a lively band and Christmas-clad stilt walker dancing through the aisles, sponsored by a local candy company. They were performing either bomba or plena, two drum-based local musical traditions that grew out of African slave drumming. Perhaps someone more familiar with the traditions can add more details and clarify exactly what they were performing?