What kind of pellets were you shooting? (And thanks a thousand per cent for mentioning the bit about eye protection. Can't begin to mention the number of pictures with members' kids shooting without any.) Lead will usually either congeal into a lump or splat but, that close, you might be picking up some bits and pieces. Hard pellets will be the absolute worst. Hope you weren't shooting PBA's or similar hard alloys. A nice steel plate, maybe 1/4" thick, angled at 45 degrees into soft material will prevent 100% rebounds like the one you experienced.
Two words- DUCT SEAL!!!!!