Horse people have a high degree of paranoia, and are almost obsessively compulsive about certain things - one of those is always closing gates after you.
There are very good reasons for this as a friend of ours discovered this weekend. She took her horse down to the fields through a side gate which she latched open to get her horse through. A couple of minutes later we heard her screaming her horse's name as he reappeared through the side gate, on his own, minus his head collar. Fortunately he just came up to see our two horses on the yard and stopped. But it could have been very different - there were no more gates in between him and the road :(.
Apparently he'd slipped his head collar as she'd gone to let him out in the field, and turned and legged it back up the track to see our two. If the side gate had been closed it would have been fine. He'd have been stopped there, but because it had been latched open, he had free access to the road.
Always close gates after you if you're dealing with horses.