This is a great question but it is rather broad. There are many reasons why there are disputes. Therefore, it is not possible to name all the reasons for the disputes. For example, some unions may want more money or better bonuses or better hours or more benefits. As you can see the issues are various. In the end, the certain thing is that there are conflicts because groups are not happy with the status quo. They want change.
In terms of means of resolving them, there are are few ways this can be done. The easiest way to resolve anything is when both parties agree on some compromise. In other words, both sides see each other's points and they come up with a resolution that both sides can agree on.
If this does not work, then the union has other avenues, because they can strike. If they take this route, they can cripple the company that they work for, because nothing will get done. Productions comes to a halt. We call this collective bargaining, because the union speaks in one voice for the sake of many. When this happens the union has a much greater voice.
In the past, America was really a land of unions. However, in more recent times, unions have gotten much weaker.