Maybe the characters went to a location they weren't suppose to go yet.
When I set up an adventure I write up an area using an matrix story structure. The characters can do what they want within the fdramework of the matrix, but if they go into an area of the matrix story that is beyond their ability that is their own damn fault.
For example I had a player go off solo to investigate a tower. All the other PCs told him not to do so, but being a stubborn bugger he went. The tower door was open so he went inside. Came across a locked dow so he tried to break it down which woke up a 20 HD house mimic that was the tower. The 1st level wizard was promptly eaten. The rest of the party went to the area of the matrix story set that was geared for their level.