It would appear the flight in question did a "return to gate". That is to say that it pushed back from the gate at 7:15, then for some reason returned to the gate around 20 minutes later, without ever having made it to the runway in the interim. Around 15 minutes later it once again pushed back, and this time continued it's flight as expected.
Returning to the gate after pushing back isn't common, but it's also not uncommon and could be caused by anything from a minor mechanical issue, to removing a passenger from the flight for any reason, or possibly even to allow another passenger to join the flight (unlikely, but it does happen!).
In this particular case, it appears that the issue was due to a "weight and balance" issue. Some passengers were moved to different seats on the aircraft after it returned to the gate. It's also possible that some cargo was offloaded, but given the short time that it spent back at the gate I'd say that's unlikely.
Aircraft are required to have their "weight and balance" numbers available before they are able to plan their takeoff, and normally the pilots would have these details before they push back from the gate. Occasionally for various reasons they will not, in which case they may chosen to push back and start their taxi to the runway (or hold somewhere nearby) in order to free up the gate and get closer to the runway for when they receive them. In this case it's likely when they got the details they found there was an issue, and thus the need to return and re-balance either people or cargo.