This is why it's best to used the same color thread in the top and bobbin. I have 2 guesses. One is that a bit of lint may have developed in the tension disks. I have also noticed on occasion that as the bobbin gets low, the tension gets thrown off a bit.
Remember, however, this is a wall hanging. If the top thread is showing slightly on the back, who will know??
There is always a black sharpie.