Let's work this backwards. Team A got a first down, so they must have been awarded it by rule.
1. Team A could have gained the first down through play. In this case you would have a run (or pass) to B-35, followed by three team A dead ball fouls (before RFP) to end back at A-20.
2. Team B could commit a foul with a first down component. One option is the DPI jg-me described, others are personal fouls, so you could have a situation where the play ends at A-35 with a live-ball team B PF followed by two team A DB fouls (again before RFP). Or team A throws an illegal forward pass from A-5 or A-10, followed by a DB team B PF.
3. Or, you could have team A legally kick the ball on second down, ball crossing the neutral zone, touched by team B and recovered by team A at A-20.
4. There could be a touchback in team A's end zone. jg-me has this covered.
Can't think of any other way for team A to get the first down. Waiting to hear what really happened