Did you cut any wood fiber or just compress it with the knife?

A sharp blade is important when doing these things as you avoid having to press hard to make the cuts.

The say of: "There is nothing more dangerous than a dull knife" is true. Being dull causes you to use excess force for a task instead of making clean cuts. Single edge razor blades are cheap and disposable.
http://www2.towerhobbies.com/cgi-bin...&I=LXL448&P=ML and probalby cheaper at Home Depot.