2 things that must happen for the LA Chargers to trade back in draft
2. The LA Chargers have Alijah Vera-Tucker (or others) over Christian Darrisaw on their big board
If the LA Chargers are high on Christian Darrisaw then it is unlikely that they would trade down from the 13th pick (assuming Darrisaw is still available, which he should be). It makes sense for the Chargers to be high on Darrisaw, as he would instantly fill the hole on the left side of the offensive line.
But there are other offensive linemen that the team could get excited about. Alijah Vera-Tucker has better game tape than Darrisaw but he is a bit undersized. If the Chargers are bullish on his ability to play left tackle in the NFL then perhaps they could value him over Darrisaw.
There is also the possibility of them being high on someone such as Teven Jenkins, or if they really want to trade far down in the first round, Liam Eichenberg. All this goes away if they love Darrisaw. If they love any of these other guys then it becomes a conversation.
If the Chargers do not draft Darrisaw then the Minnesota Vikings likely will (or at least should) with the next pick. The Bolts could trade to 15 and absolutely still get AVT. They could trade down to 19 or 20 (Washington or Chicago) and still likely get Jenkins or they could trade all the way down to 24 (Pittsburgh) and still get Eichenberg (or whoever they love, really).
If there is a quarterback available at 13, and the LA Chargers draft someone other than Christian Darrisaw, then it will be a huge missed opportunity for the Bolts.