As you stitch, you add a subtle twist to the needle every time it moves through the fabric. This causes the thread to twist, and eventually a twist will fold over on itself. Then you stitch over that, causing a knot.

The fix is two fold: First, let your needle dangle ever 10 stitches or so so the twist drops out of the thread. Second, check the back of your fabric frequently to make sure you're not getting a tangle. I like to hold my fabric so that the thread runs under my finger as I pull through to the top, that way I can feel if a knot forms.

Well said. If you catch it before you make the next stitch, pulling on the loop usually releases the forming knot. If you use flat bobbins to roll thread, that can cause creases/bends in the thread. Those bends are where the knots try to happen. After I load my needle with thread, I moisten my fingers and run them along the thread until it relaxes. Getting rid of those bends should eliminate a lot of the attempted knots.

Is it going too far out on a limb to guess that maybe you like pink?