Hello,
First of all let me ask, did you build that deck? If so you are better than 3/4 of the "pros" NICE BUILD! Thank you for not using 4x4's to hold your deck up. Second docchop1 is on the right track. Going by how well the gate is built and what you are saying and what I see, the stringer that the hinge post is attached to is rotating do to gate weight, not uncommon. If what you did is working for you and it doesn't happen again than you are set obviously, however if it happens again you need to get that hinge post to stay plumb. The top of that stringer is the pivot point the bottom edge is swinging to the outside causing the top of the post to swing in "toe in" toward the other post. You need to link the bottom edge of the left stringer to the bottom edge of the right stringer with a 2x6 on its flat screwed to the underside. This will link the bottom of the hinge post to the latch post.
