You have to remember, Amy is just a bit better with personal relations than Sheldan. He experience with sex and relationships is almost null. So with Leonard gave her a peck on the cheek, she took it to mean more. Imagine her as a blossoming teenager, a teenager will read any sign of friendliness as a come-on.
Don't think that Bernadette was suppose to be imitating the mother, in another episode she yelled out to the mother the same way. Think that in the show, when she yells she has the same characteristics as Howard's mother. Something along the lines that men, especially men who have heavy attachments to their mothers, tend to find woman that subconsciously remind them of their mother . (Men marry their mothers, woman marry their fathers)
http://www.sciencedaily.com/releases/2008/05/080505223427.htm