I learned in the past the value of having an extra adventure or two rolled up on the side in case my PCs wanted to be unpredictable. Sometimes PCs just don't move around the way you want, or ignore your adventure hooks. That is fine, part of the enjoyment.
The way I usually "hook" side adventures is to have a note somehow find its way into the party's hands. Be it through combat, message in a bottle, mistaken identity, whatever.
So the PCs just get hold of a bandit's orders (after killing him, obviously) only to find it is detailing the routes they plan to ambush. The Ranger is naturally infuriated, and storms off shouting something about it being his duty to protect the innocent. The rest of the party, unsure of what to do, chases after him to help.
I should take a moment to say a lot of my events are timed. I keep track of the passing of days in my adventure, and have things running all over. For instance they may find a caravan looted with dead bodies around, which may lead to a quest. Conversely, if they reached that area a day sooner, they might've been in time for the ambush. Simple stuff like that.
Back to the party, they burst out of the forest and come upon the caravan, and find several dead bodies with some men standing over them, seemingly inspecting the dead. Before the men even stand up to speak the Ranger immediately starts dropping them with his arrows. The rest of the party is like
O.o
At this time, the men rally to fight back and push to a pause of sorts in the combat. They start shouting they are city guards, coming to investigate why the caravan never showed. Ranger says,
"Oh "
After a long conversation and many many charisma rolls to defuse the situation, we now have a disgraced Ranger (now fighter) and the rest of the party accessories to a heinous crime. So after a brief stay in the dungeon, they are offered a shot at redemption by undertaking a harrowing quest to clean out an Orc hideout near the city.
I have a new lesson in the unpredictability of PCs, and I think they learned questions first is usually a safe bet.
On an aside, once Rangers commit a crime like that, they lose Ranger status forever. Do you think I should let him do a solo quest to regain his honor and title? Or forever damn him to his actions?