Caveat: I'm not defending Talib or his post-game twits.
Having played the game before, I can say that it just happens. I'm sure he wasn't planning to grab Brown's face mask. However, he was playing in one of the biggest games of his career, backed up to his goal line and he got beat. I think any of us would have reached out to grab anything we could in order to stop the ball carrier and we would have held on until we knew he was down, short of the GL. Just like Bosa wasn't intending to stick the crown of his helmet in Kiser's chest (and bounce off of him instead of making the tackle).