Here is what I was told many years ago - - by Rod McClellan at his nursery in South San Francisco. It was his story that made me into a Bulldog Lover..
First you have to understand that the English were the big discoverers of orchids, the first to breed them, show them, award them. Then you have to look at the shape of the original bulldog crosses - the ones that caused the nickname to be given. And you have to remember that the English Bulldog is the pride of England's dog culture.
Bulldogs have a rounded, jowly face that imitates (if you squint really hard) the form of the classic bulldog paphs. Add to that the fact that the likely most classic bulldog paph is the Winston Churchill, and it all rolls into bulldog becoming the accepted name of a given group of paphs...
Bulldog paphs have large, waxy, shiny rounded blooms. They are very longlasting blossoms which may also have contributed to the name (holds on like a bulldog) As hybidizing continues, there are many now that do not have the round form, but that style of large, waxy single bloom paphs all get called bulldog.
Any other explanations out there??