When I DM the players are sometimes absolutely predictable, and sometimes insanely unpredictable. Usually due to how they have specific habits and reactions to certain situation or NPC personalities.
Due to the unpredictability and my own preferences I tend towards a bounded sandbox setting/scenario style of game. However, when I can predict some/most of what the players will do, I try to manipulate them into situations where they have to choose what & how much the thing they want is going to cost. Generally only statting things out about a session and a half ahead, although I often have semi-detailed setting outlines and a stock NPC backup cast.
I have been accused of railroading when I successfully predict their actions and they get faced with choices or consequences. Although I think its interesting that I get more accusations of railroading when they get consequences of their actions that arise from the insane random crap they sometimes pull. Maybe I make a weird mirror reversal of railroading somehow. They seem to feel that physics/NPCs reactions to their lol-random detours are railroading, while if I plot 2 paths with 4 encounters each that lead to the same end scene it isn't a railroad.