The sonar was too low frequency and too powerful and I guess it messed with the whale's echolocation (like blowing their eardrums and maybe causing brain damage or other problems) which caused all the whales to get beached and die. It could have also caused compression/decompression sickness or effects on the whales. Any of these could cause them to die.
Almost like someone pounding on your chest with a really powerful wind- except sound travels better through water, and water is much heavier.
You can probably imagine whatever it did to the whales, it couldn't have been good.
Beamforming only affect beams they overlap with. The sensors left and right get farther away from the center and their power dissipates too quickly over the distances it takes for their beams effectively to overlap with the wave from the center transmitter. It's not like a magnifying glass where you focusall the light rays to a single spot. The energy in the parts of the wave that do not overlap with the center wave is lost. Even where the wave does overlap, a lot of energy is already lost from dissipation through the water.
Frequency also affects it a lot so for longer range you would need to use a very VERY VERY VERY low frequency."
Pressure waves (like sound waves in water or air) dissipate energy much more quickly with distance if they are a higher frequency. As an example, for a two sounds waves of the same power:
120kHz- travels ~1.5m, beamwidth is 15 degrees
40kHz- travels ~6m, beamwidth is 30 degrees
A high frequency wave loses energy much faster, but is much narrower. A low frequency wave travels much farther, but is much wider. So you use a low frequency sonar to see far away at a wide area, with low resolution or you use a high frequency to see very close to you with lots of resolution. It's like if you use your eyes to look far away you can see a very wide area with little detail and if you look at something in your hand, you see a very small area but lots of detail.
Think about this: When some jerk is driving down the street with his speakers turned up really loud, can you hear the words of the song (high frequencies) or can you only hear the thumping bass from the subwoofer (low frequencies)?