You are pinching some of the tube between the edge of the tire and the wheel. One needs to carefully go all the way around each edge, and push the tire in a little and be sure you can see metal. Only then do you put more air in the tube. The long rip and delayed explosion are typical symptoms of this. It is not unusual, lots of new tire changers do this. Practice and patience will help. Inflate the tube just a little to get it up into the tire. Then as you get close to the
last part of the bead to go over the rim, deflate the tube a little. Pump in a few psi, then visually compare the line all around the tire on both sides that is near the wheel. If one section of the line moves away from the wheel that is a sign that there is pinched tube in there.