Another possibility may be that Barnes harbored some kind of long smoldering grudge against disantis, who appears to be allied with the progressive wing of the democrat Party. Being older and experienced, Barnes might have been resistant to witness coaching from cross and co. Apparently in spite of his extensive legal and political experience, Barnes seemed to get all chatty Kathy on the stand including the bits about disantis. As they say, nothing in politics happens by chance, and there just may be divisions within the democrat party similar to divisions within the Republican Party.
The thought had crossed my mind.