I'm a pretty relentlessly rational person, on the whole, so it always irks, or at least amuses, me when I notice myself doing something completely irrational. (Not to be confused with unreasonable, mind you -- I am highly unreasonable on a regular basis.)
In that spirit, here's a behavioral quirk of mine that I was thinking about the other day: I always touch the outside of an airplane just before boarding.
There is a rationale for that, in a sense. If I'm about to trust my life to a machine or tool, I'd want to give it at least a cursory inspection first. Boarding an airplane certainly qualifies, unless you're just planning to take it for a spin around the tarmac. But you and I have neither the ability nor the opportunity to gauge the flight worthiness of a commercial jet beyond observing that the correct number of wings and engines are present. So somewhere along the line I picked up this habit of touching the fuselage on my way through the hatch. Is that a "I'm trusting you, so don't let me down" pat like you'd give a horse? Just making the only hands-on inspection I'm allowed? Couldn't say.
So what's your quasi-rational quirk?