פירוש על קידושין 66:13

R. Shimon b. Abba describes himself as only a “haver.” In this context it seems to mean that he is less than a sage. This is the first reason that they should not rise in front of him. The second reason is that since they are actively engaged in studying Torah, they are Torah itself. He, on the other hand, is just a student of Torah.
It is admittedly a strange story. Perhaps it shows some discomfort that sages felt with having other sages stand in front of him.