OK, I now have an answer from William of Conches (a 12th century writer; this is from A Dialogue on Natural Philosophy, a.k.a. Dragmaticon Philosophiae). I hope I can summarise correctly:
There are a number of ways to think about the seasons, and there is no consensus over the cause. What is evident is that the sun (and the other planets) appears to travel from west to east over the course of the year, and daily from east to west as it rotates around the earth. In other words, throughout the year, the sun appears to travel from Aries to Taurus to Gemini, etc., which is a movement towards the east. (Note that the other planets are also observed to make the same procession).
Those loyal to the Peripatetic explanation claim that the sun only appears to travel towards zodiacal signs in the east; but in actual fact the signs were moving towards the sun. For example, if the sun rose today in the first part of Aries, the firmament and the sun go around the earth in a circle the whole day and the following night, but because the firmament is faster than the sun, the first part of Aries, which rose today with the sun, will rise tomorrow before the sun. It therefore appears that the sun has moved into a more eastern part of Aries
William of Conches prefers an explanation he claims is more in line with Plato. He says that the sun really is moving from west to east over the course of the year. This is contrary to the movement of the firmament (the sphere of stars) which shifts from east to west daily. If the planets and the stars were both to move in the same direction, "so great would be the impact on the earth that nothing would be able to stand or live upon it." The daily motion of the sun around the earth is due to the turning of the firmament which draws the sun and the other planets with it.
In either model, the sun and the other planets do not move in a straight line from west to east, but obliquely (i.e. not at right angles to the horizon, or parallel with it). When asked in the dialogue why the sun has this oblique movement, William of Conches answers "so that it could provide us with the four seasonal varieties of the year". So, not so much an explanation of the seasons, but rather an explanation as to how God caused the seasons to occur. Winter begins when the sun enters Capricorn; because then it is most distant from our habitable zone on the sphere of the earth, we are frozen with cold. When the sun reaches Cancer it is closest to us, and its heat and dryness scorches the earth. In spring and autumn the sun is neither too close nor too far, so the seasons are tempered.
Interestingly, William also provides two possible explanations as to why seasons are not identical every year, but vary in weather. Some philosophers claim that the other planets produce their own summers and winters. so if the winter of another planet occurs during the summer of the sun, our summer is less hot and dry, but if the summers occur at the same time, the summer of the sun is even hotter and drier. Others contend that it is the innate qualities of the planets that cause the variation; so if a hot and dry planet is with the sun during our winter, then the winter is mild.
I can only apologise for a lengthy explanation to a simple question! I hope this helps.