That same thing always happened to me.
My horse would always act up when saddling.
My horse trainer told me before we took my horse home to talk to the horse whenever your by her or him.
If I do any fast movements near my horse or on my horse she bolts. So I did what my trainer told me. So talk to your horse and don't be quite around, because maybe your horse doesn't know what's happening and spooks. Same thing with the homeless people:) haha. Pet your horse and reassure her that the homeless guy isn't going to hurt her.
Horses are stubborn sometimes when saddling. For some reason they like to breath out when your tightening the cinch. Then when the breath in or however you would say, when you get on and therefore the cinch isn't that tight when they breath in and the saddle slides. Well I hoped this helped you!