First of all, they are Kerbals It looks like you're pitching up way to much when you're re-entering making the plane pitch up even more because of the lift that your wings suddenly have, entering the atmosphere. Also never use the physics warp/time warp because it messes up the physics. Another possible solution is to balance or fill your plane's fuel. I suggest this because the fuel usually drains from the front of the plane again, making the plane become unbalanced and more prone to tipping upwards.