There's an interesting report that analysed the activity of the Russian troll social media accounts that have been publicly identified.
It turns out that most of their activity involved promoting (commenting, retweeting, etc) stories that appeared in the main stream media. One of the top media organizations the troll accounts referenced was Breitbart News.
My guess is Mueller is grilling Bannon over his time at Breitbart, whether Breitbart had contacts with these troll accounts, and whether any of that bled over into the Trump campaign when he joined in Aug. 2016.
Good guess given Mueller's announcement of indicting the russian bloggers.