Why did Barnes Wallace decide to spin the dambusters bomb backwards?

The backspin was used to ensure contact with the dam underwater.

If they imparted forward spin on the bomb it would hit the dam and shoot skyward when it hit, just like if you rolled a bicyle tire at the curb, it wants to keep going.

Wallace reasoned that in order to get the bomb to counteract the forward velocity of the aircraft, the bomb would have to be spun backwards at a rapid velocity during flight. He also reasoned that would have to calculate (using the drop distance from the dam, the forward velocity of the aircraft, and the elevation of the aircraft), how many bounces the bomb would achieve on the water before reaching the dam, and on each bounce, how much drag the bomb would incur as it scooped through the water on each bounce. Using these specific factors in his calculations, he was able to fix the velocity of backspinning required (almost 600 revolutions per minute) to enable the bomb to slow down on each bounce to the point that the bomb would almost be at zero velocity at the point that the last contact with the water was made- occurring right adjacent to the dam.

As the original author of this answer correctly pointed out, the backspin then guaranteed that, if it had any forward velocity left at all, the bomb would then simply contact the dam face, roll down the wall face of the dam to a depth of 30 feet where an internal pneumatic pistol ignited the charge causing the bomb to explode hard against the dam surface.

Although the backspin would allow the cylindrical bombs to "crawl" down the dam face if they came to rest exactly at its surface, there is evidence that Wallace also chose back spin to take advantage of a more subtle aspect of physics - Kutta's lift theorem. More commonly known as the Magnus effect, Kutta's theorem explains the unmistakable curving trajectory of a spinning baseball. Given the operational uncertainties of the attack - night time, altitude variations, crude aiming devices and monumental distractions - Wallace had to appreciate the fact that most bombs would likely strike the surface of the dam on the lake side, bounce off, and sink some distance away. Archival footage of the Royal Navy version of the weapon actually shows these bombs bouncing off the side of a ship. Once the heavy, spinning bomb begins to sink, however, the upward flow of water around the cylinder, coupled with the back rotation, generates a truly impressive hydrodynamic force pulling the bomb back toward the target - the Magnus effect. In the end, the bomb needed to be spun in order to stabilize its path to the target and to prevent it from tumbling out of control. There was no alternative. Sir Barnes Wallace, through his meticulous experiments and his understanding of physics, was able to select the direction of spin that would maximize the bomb's effects.