The Shulchan Arukh (OC 663:3) says that on Shabbat of Chol haMoed Sukkot (as with the Shabbat of Chol haMoed Pesach) we read from Exodus 33-34. This is learnt out from a baraita in Megillah 31a, although no reason is given. The Chofetz Chayim (Mishna Berurah, 663:8) says that it is because there is a reference to the sukkah within this passage ("שיש בה ענין סכה").
Where is the reference to the sukkah in Exodus 33:12-34:26?