1. SWAT team escorts either hostages or terrorists away from the scene.

2. Bus with terrorist (no hostages) drives off.

3. BTR stops bus.

4. Guy runs up to bus, hits it with contact explosive, blows open back of bus. SWAT team jumps off BTR while this is happening.

5. Dude on bus has no idea what's going on, SWAT teams rush bus, take out terrorist. Everything is done.

That's not a lethal explosion (you can see the guy getting up at the end), but you wouldn't use it on a bus full of hostages because you'd definitely do some damage.

If this were in real life, it would probably have something to do with hostages being traded for a bus (or hostages released off bus in turn for a getaway) and the hostage taker driving away. On down the road, ambushes are set up to stop the vehicle (that no longer has hostages in it).