Most of the times Bruce Banner became the Hulk, it was from an alien falling through his roof or a stray laser hitting him in the head. But comics never showed us how meek Bruce Banner dealt with his normal, everyday nerd rage. Nerd rage is a phenomenon where impotence collides with anger and frustration to create a whirlwind of cushion-punching action! They feel it when the Internet calls them a f*g, an hour after someone cuts in line or when Ninja Gaiden just won't f*****g listen. I always wondered what Bruce Banner did during times like these, and that led to: Hulk's Incredible Tales of Nerd Rage.
Sometimes it's just a matter of making the US Department of Defense look, like, REALLY cool.
Actual impending doom like global climate change or mass extinction just makes people bored.
In some cases, the Marvel source material just did better.