Thanks for the question. We sort of alluded to it in this post (https://www.climate.gov/news-features/blogs/polar-vortex/welcome-polar-…) but the seasonal cycle in stratospheric winds  are strongly controlled by the seasonal cycle in incoming solar radiation. When the sun returns to the North pole at the spring equinox, the stratospheric temperatures start to warm, enough so that the difference in temperature between the pole and the equator changes sign (it becomes warmer at the pole than the equator in the stratosphere during summer). This means that the winds must also change direction to flow east to west in summer. However the timing of when this happens in the spring varies a lot from year to year- we will explain why in an upcoming post. Same thing happens in the fall- as we approach the fall equinox, the temperature difference in the stratosphere again changes sign as the pole loses sunlight, making it colder there than at the equator, and the winds turn west to east once again.