Here's a really good article by Grace Collier on union culture which has some useful insights. She says that many unionists have a war mentality and therefore believe that pretty much any action can be justified as long as it can be seen to benefit the tribe. She also likens the atmosphere of many unions to that of a cult. If you keep in mind these characteristics you can see how easily corruption can flourish.
There's another aspect of unionism that increases this tendency. That is that you don't actually have to produce anything of market value when you're in a union. The money flows into its cofffers in a steady stream from the pay packets of employees no matter what. It's like taxes being used to pay government employees. It's pretty much guaranteed.
That's the opposite of being in a business -- particularly a small to middling one. In that environment, if you don't make sales, you go belly up. So all or most of your attention is directed towards that goal. You don't have anywhere near as much time to play politics.
But in a union, it's all politics. You're playing politics with the employer to increase the benefits to members. And you're playing politics with your fellow unionists to get to the top of the power structure.
If you think of these three factors together you start to understand what a strange world these people live and work in, day in and day out. Behaviour that most people would find dodgy as is often seen as pretty much par for the course and therefore totally acceptable. It only worries them if they feel they might eventually get sprung for it.