Welcome to Gaia! ::

I know that the Pink Floyd one is intentional. I don't know about the rest.
I think most of it is just power of persuasion. If you tell people what to listen for, they're more likely to hear it.

Sometimes it's just like clouds in the sky. Some people see the bunny, some people see that cute puppy, and others see Satan. It's all about perception and the power of suggestion.

Sometimes it's intentional - as phriedphishphan stated.

Speaking as a musician, even if one were to intentionally record something that sounds backmasked, it would be a tiresome process to mix it into a track. True, since .wav editors were invented, it's gotten a lot easier, but most allegedly backmasked music was made in the 70's and 80's when a computer was a big boxy affair sitting in some government basement someplace and not a happy friendly box full of consumer-grade electronics sitting pertly on someone's desk.

Conclusion: early backmasking was inadvertent, but later on, when a panic ensued amongst the not-very-bright-parents-of-depressed-suicidal-kids community, some bands decided to do it just to be smartasses.

Hmm... maybe the next Turbosloth record should have some backmasking...

Oh yeah, I don't think backmasked stuff is at all intelligible to the human brain. You might as well say two depressed Judas Priest fans heard the wind whistling in the trees and thought it said, "You know what? You two really ought to shoot each other fatally with some sort of really intense firearm".