When cooking pasta you must remember to not only watch it for its al dente texture (no more, no less), but also to use plenty of boiling water. Pasta contains a lot of starch that becomes very gluey when cooked, so you need to dilute and reduce the pasta's tendency to stick together. When you throw in pasta into the boiling water, the starch granules start to absorb quite a lot of water and therefore they swell and become sticky. A good amount of starch also becomes released into water (as you can see after you've cooked the pasta) and can act as an extra glue between the pasta pieces. That's why you need a lot of water to dilute this effect.