are they full featured sites that can easily stand on their own?
If you don't have 'gateway pages' (and only you know for real, we don't) 20 sites probably seemed too much to Google and maybe thought that you're trying to exploit a domain name match, interlink or whatever. Just a guess.
But Google has been going after people with many sites, not that it's illegal but usually quality lacks when one has dozens and dozens of sites.
And using one WMC account for all is probably not a smart idea, I have heard that they check the entire lot when they suspect one.