הלכה על בבא קמא 163:4

Ezra the Scribe further instituted that the Torah be read at Minḥa on Shabbat because of  “yoshvei keranot” (BK 82a). Some say that yoshvei keranot are merchants and craftsmen who are unable to hear the reading on Mondays and Thursdays, and so the Shabbat Minḥa reading was added as a make-up opportunity for them. Others maintain that yoshvei keranot are frivolous individuals. There was concern that after they finish Shaḥarit on Shabbat they would turn to drink and idleness, and so Torah reading was instituted at Minḥa (see Peninei Halakha: Shabbat 5:8). The Sages further enacted that relevant Torah portions are read on all festivals and Rosh Ḥodesh.