Board Thread:Episode Discussions/@comment-25319479-20170426101051/@comment-30323490-20170427205512

Boruto showed interest in summoning a snake since the beginning of the episode, so when he had to chose a contract scroll during the capturing flag thing, he chosed the snake one.It has nothing to do with Mitsuki. Also, Boruto would've probably summon a very small snake, (similar to Naruto with the small toads) if it wasn't for that thing that is shadowing him and the others. So when that thing intervened between Boruto and his summoning seal, Boruto probably unknowingly used that thing's chakra, hence why the snake was so big and evil (and obviously displayed the thing's chakra).