Stranger on stranger crime is an opportunity affair. It's like what's shown on those nature shows where the lions seperate their victim from the herd. The criminals try to draw you away from your comfort zone and pull you into theirs. They can ask for directions, a few bucks, a light of a cigarette or anything in between. All of it is a ploy to break you away and make you vulnerable.
When OP was slowing down and taking a sip of water, he was like the wildebeast with the broken leg. The predators sensed that he was vulnerable. Lucky for him, he had the strength to be able to take off and get away.
BTW: Non-stranger crime is the #1 problem in this country. Friends, lovers, and spouses blowing each other's brains out is much more common than stranger on stranger crime. That is the primary reason I wll never own a gun. I simply do not trust myself enough to know that I would not fall into that category, especially in a fit of rage or after a few cold ones.