It is natural for evergreens to shed a few leaves in the autumn, however my immediate thought is that it may have outgrown its container as Bay trees have quite extensive root systems. Try re-potting it into a larger container in the spring, using a soil-based compost such as John Innes number 3. If the rootball is compacted then loosen some of the roots with a garden fork before repotting it. The yellowing leaves could also signal a lack of nitrogen in the soil, which is essential for healthy leaves. In the spring try feeding the Bay with a well balanced liquid feed, or work some fish, blood and bone into the compost when re-potting it. Continue feeding it once every 2 weeks throughout the summer. One final point, Bay trees are very resilient to drought but will be much happier and healthier if they receive a regular supply of water. This is particularly important during hot dry periods when the soil can dry out surprisingly quickly.