This started at the same time that top-tier income tax rates were brought down from their 93% peak in the late 1950s. Turns out that when you find yourself above that line, you take efforts to actually spend money on things you can deduct, rather than buying up shares that you can't deduct. When you find yourself with profits $10,000 over the line, and you're told to send $9,300 to the IRS, you tend to say "Well, fuck that, I'd rather pay out $10,000 to the guys who brought me up to the line, rather than $9300 to the tax man."

If you stiff your workers on that excess $10,000, you only got to keep $700. If you fuck over the workers who put you $10,000 above the top bracket, you get $700. It's worth a lot more than $700 to keep those workers happy, so you pay out the excess as a bonus, and bring your net income back under the line.

Now, though, it's not a $700 reward for fucking over your workers. It's $7000. If you have $10,000 over the line, you can fuck over your workers and keep $7000, or you can pay out $10,000 worth of bonuses. Given those options, they all fucked over the workers.