[RPG] How do “monsters” attack if the PCs don’t engage in combat first

I had this problem at first too. Here are some good uses of minions.

1. Archers. Long range archers. This was the first effective use of minions I found. Make it so that rushing over to kill the minions is a time investment. Better yet, keep the minions separate from each other so that each individual minion is a time investment.
2. Terrain. This is a corollary to archers. Minions cost XP. Terrain doesn't. If you put your archer 8 squares away, the PCs can get to him. If you put your minion 8 squares away, in a tree, with a moat, the PCs can't.
3. Don't use minions for damage. There are a number of minions that invoke penalties just by being present. Wind Stalker for instance slides creatures that start their turn adjacent to it.
4. Or use them for lots of damage. The elemental spark minions in particular can explode when they're killed to hit a burst of PCs. Position them correctly and the players won't be as interested in killing these guys so quickly.
5. Use higher level minions. This is counter intuitive since I started out filling up the leftover bits of XP with minions. But investing XP to get some good minions can be worth it. Minions that are a couple levels higher than the players, particularly if they're soldier minions, will be harder to hit and they'll be capable of hitting the players. When I started using minions that were several levels higher than the party, the PCs started assuming that the normal enemies were actually minions because they seemed weaker by comparison.
6. Misdirection. This relies on 5. If your players don't know which enemies are minions and which aren't you can trick them into wasting their encounters, dailies, and action points to pop minions. Using minions that are more powerful than the NPCs (as in 5) is one way to do this. Another is to throw in one or two unique minions, instead of just tossing 4, 5, or 6 identical guys down there. Give your unique minions good minis and a bit of description. Not only will the minions eat the party's good powers, this will also keep the real boss alive longer to take more actions.

Introduction

A miss means that the character's action is unsuccessful or carries major consequences. Unless the move tells you what to do, all moves work the same on a miss—the GM takes action, doing something dangerous to the characters.

Chapter 12: The GM

You also make a move when the players give you a golden opportunity. A golden opportunity is any time they ignore a threat or when they fail a roll (6-).

When they give you a golden opportunity, you can make your move just as hard as you like. A hard move is one that is irrevocable and immediate. The players immediately feel the consequences of the move and have to deal with them. Dealing damage is a hard move, since the damage is immediately applied.

On a miss, the GM makes a move. In particular, the GM is empowered to make a hard move, which includes just dealing damage! Of course, the response should be guided by the GM's principles, including make a move that follows (in the fiction). If you miss Hack and Slashing an orc, then it would follow for you to get hit by the orc. It wouldn't necessarily follow that a cave-in occurs, unless there's been some buildup in the fiction that a cave-in is about to occur.