The first time Taeyeon patted Sehun on the head during "DJ got us falling in love again" performance was because the both of them messed up, they were about to run into each other getting into their position.

August 05, 2012, SMTown concert in Tokyo Dome, Taeyeon pats Sehun's head again...

Okay you two...what's the reason for head patting again? Did you two plan it all out to "PURPOSELY" bump into each other so that Taeyeon can pat Sehun on the head just so she can show everyone who Sehun belongs to?

LOL. Oh you two~

QUIT KILLING ME HERE!!!!! I CAN'T TAKE IT ANYMORE!!!!