Science Fact: When men are romantically interested in a woman, they unconsciously start using more showy, unusual, or creative language.
Explanation: You might notice yourself doing this, but it's more likely that you'll notice when someone else does it. Let's say you're hanging out with your buddy, talking about orkz or salarians or some other topic that would make the average woman start backing slowly toward the nearest exit. Suddenly, from stage left, a hot girl enters the conversation, and your buddy promptly starts talking like a broken thesaurus. "Pardon my blandiloquence, but I find you positively pulchritudinous," he says to her, hamming it up like he is made of ham. "Yeah, you are, like hella pulchrimus," you find yourself saying, for some reason. "If you pardon my... bland...donculence." Oh my God, I must be having a stroke, you think to yourself, as your buddy asks the girl if she wants to go osculate in the broom closet.
Now, if you were alone, you probably wouldn't even notice what you were doing. You would just sit there happily telling this girl that she is beaudiculous. Only to an outside observer, it would sound like you were going "blehhh" and barfing out unrelated parts of a dictionary.
The Science: To a point, this behavior can be explained by the fact that girls make us stupid, but there's something else at work here, too. One study started out by priming college-age guys and girls to think either socially or romantically. Some participants were given pictures of older (opposite-sex) models and told to pick the one who seemed friendliest, and then to write a description of hanging out with that person. Others were given pictures of models their age, told to pick the hottest, and then to describe that interaction. Naturally, guys in this second group went "Girls?? Hot girls?! I AM SMART AND STRONG," and proceeded to shoot grammar out of every orifice. If they were primed to think about sexytimes, they used language primarily to show off, much more so than the group describing regular social stuff. (The theory is that we unconsciously use big words to demonstrate that we are smart, desirable mates, more genetically fit than someone whose pickup line is "ME YOU CREATE A BABY.")
Interestingly, for women, it's the opposite; girls primed to think romantically used fewer unusual words than the ordinary/social group. This is in keeping with the generalization that males do the showing off, and females do the choosing between them; language is just another way we strive desperately to get noticed, like a peacock showing off his plumage or a giraffe riding around on a motorcycle.
So What Should I Do About It?
Nothing. You should keep doing it.
Of course, it certainly helps to be aware you're doing it, and to do it correctly—stringing together a nonsense tirade of bizarre words like a mutant Shakespeare isn't going to impress anybody. But the main reason we engage in these verbal acrobatics when confronted by a beautiful woman is that such behavior usually works. When pursuing someone romantically, women are more impressed than men by eloquence (and status, and intelligence, and some other stuff). So you may not want to go around telling the ladies that they're orchidaceous, but this isn't an issue you have to lose any sleep over. Just keep anything too stupid from falling out of your word-hole and you'll be fine.