Think tank is an expression often used in the creative sessions of various businesses to come up with new ideas, like advertising campaigns, new products or solutions to business problems like how to make money or how solve problems created by competitors. Usually the problem is tabled and everyone gets together in a room to come up with creative ideas they usually have the rule that no idea should be censored as other ideas often come from the unusual.
I would venture to guess that it just seemed like an appropriate name for that technique 
Alex
