It's a plain and practical matter.I hate the assumed connection between plot and railroading.
If by "plots" one means no more than the best laid plans of mice and men, with no more predestination than evident in real life, then -- as in real life -- "story" is simply an account of whatever happens. There is no "line" from which to deviate, and thus no "getting off track" to correct.
Not too surprisingly, that often does not produce a sequence of events in conformity with the demands of dramatic structure. Stuff happens, and in a well-played game it's pretty interesting stuff to the players at the time -- but most of it is likely to be more "you had to be there" than high theatrical art.
As soon as one introduces such a thing as "THE plot", in the sense of the structure of a novel or play, then there is such a thing as the "wrong" things happening. When a Game Master arbitrarily (a theoretically avoidable condition, perhaps) intervenes to steer events away from those and back onto the "right" course, then that is what D&Ders have called "railroading".