Let me put it this way. It's like watching a movie, and the protagonists enter a cave and are attacked by minions of the antagonist. They win and are debating amongst themselves whether they should enter the cave to confront whatever the antagonist has in store for them, or leave to better prepare themselves. Suddenly, the cave entrance collapses behind them, making their decision for them. There better be a darn good reason why that cave collapsed or else I'm going to be disappointed that the plot is being propelled by the word of the author rather than meaningful decisions on the part of the characters. Sometimes this can work, for example if the antagonist specifically traps the protagonists in his lair in order to prevent them from getting help from outside. But sometimes it would just be disappointing.
The story in a role-playing game that I run, just as much if not even moreso than that of a movie or novel, should be propelled by meaningful decisions by the characters. When they enter that cave, I want them to be able to look at their options (which include at least delving or retreating) and choose which one they want. Both have benefits and consequences, but different situations call for different decisions. And when the players make that decision, they feel in control of their adventure and subsequently become more invested in it. If the DM continuously makes decisions for the players, they lose that sense of involvement.