The Lancaster bomber will climb after bomb release, but the forces in the wing structure will be less than they were when they were carrying the bomb.
With the bomb on board the wings are 'pushing up' with a force of 57,000lbs. After the bomb is released, the wings are still pushing up with a force of 57,000lbs, but the weight they are carrying is now 12,000lbs less, so the forces in the wing structure must be less - even though the aircraft climbs, and even though the aircrew will feel positive g.
i.e. The wings do not suddenly push up more at the point of bomb release - they are 'pushing up' the same as they were.
I have watched the video explanation about manoeuvring speed, but am confused that reducing the fuselage weight the wings are carrying somehow increases the stress in their structure? How can that be ?
I think this pilot simply pulled up too aggressively, and that is what over-stressed the wing. The reason he pulled up too aggressively was that he applied a larger pitch force that would have been required with the payload on board - learned as he took off and flew the aircraft towards the venue - but he forgot to moderate his control inputs after the payload had gone.
e,g, say he would need N degrees of up elevator to pitch up when heavy; he would only need, say, a half or a third of N to pitch up when empty, but he applied N, which in the now very light plane was way too much elevator input.
If he had held the stick and not moved it in pitch at payload release, the aircraft would have climbed, since the elevator was already applying a high degree of elevator pitch-up to enable the wings to carry the payload. But then on top of that he applied even more pitch up to pull up, and this combined with the already high elevator deflection, simply rotated the (now empty) aircraft much too aggressively which rapidly took the wing AoA way beyond what it was built for, (at the speed he was flying), which overstressed the wing structure, (or the wing structure was weakened due to age or corrosion).