Like just about everything else in this world, science is about money. And how do you get money in science? By finding and/or hyping the next leap forward. Being successful in science is all about getting grants. You don't get tenure without bringing in grant money, you don't get grant money without publishing in the best journals, you don't publish in the best journals without finding the next leap.
Your typical PhD finishes school in their late 20's, probably with significant school loan debt. He or she then gets a postdoc where they can barely afford to live in the city with the prestigious school that they think they need to further their career. At the same, it's probably time to think about starting a family (especially if you are a woman). And as a postdoc, the pressure to publish is even greater in a more compressed time frame. There is so much pressure there financially, emotionally and mentally, that is it is no wonder that some people cave and take shortcuts and fudge results.
And then, if you do make it to a tenure track position, you don't do much science any more. Instead you spend all your time writing grants and churning through postdocs, who may or may not be fudging their results to get a recommendation to get a better position...