Food order panic

For some reason that I can't quite explain, I sometimes have trouble with basic decision making. I'm quite good with it usually, however on strange occasions I seem to freeze up and blurt out the wrong answer.

Know Your Meme.

The time it most frequently plagues me is ordering food at fast food establishments. I'll be waiting in line, looking at the menu and will be tossing up between two choices.

"Hmmmm, do I want a Zinger Tower combo, or will I go with the All Stars box?" I will be frantically trying to determine my choice as I'm moving through the queue, and suddenly I'll be standing at the cash register with a pimply faced teenager looking at me.

"Hi, how are you? What would you like to order?" He asks with a vacant expression that no doubt reflects his mood for wanting to be situated anywhere but exactly where he is standing.
"Hi! I'll er, have a Twister combo, thanks." What? Where the hell did that come from? I wasn't even considering a Twister...?! Before I can amend my order, he punches it into the system, and continues his interrogation.

"Would you like to upsize?" He asks. I swear I can see detect a hint of his infinite sadness behind his glossy irises. That, or he's stoned. Probably stoned.
"No, thanks." What? Again? But, you always want to upsize...?! A regular meal won't be enough. Quick, tell him you want to upsize. Alas, the moment has passed. He jumps in again.
"What drink would you like?" What is this, the Spanish Inquisition?!

Okay, so it's probably not that much of a big deal.

"Uh, 7UP thanks." 7UP?! Are you freaking kidding me? When have you ever had a 7UP? You drink Mountain Dew. You always drink Mountain Dew.

As he stumbles around like a sedated toddler trying to round up the required foodstuffs for my meal, I stand there like an idiot and say nothing.

Why does this happen? I truly can't explain it. Maybe my sub-concious is trying to look out for me somehow. More like, he's just being an arsehole.