If they did alter data, here are a few possible reasons:
They were sure they were right, so they got ahead of the available information because the science would eventually catch up with them.Of course there are many more. These assume good faith (of a sort) on the scientists' part.
Drastic climate change is so dire a threat that they altered the data so as to help prevent global catastrophe—again, it wouldn't be fraud because they'd be proven right.
They were afraid funding would disappear before they could convince the world of the need for action.
It would look as if they'd been wrong for years, and their careers would end when they still had crucial, even world-saving, work to do.