Remember that 2½ tribes wanted territory on the east of Jordan. Moses had approved this on the condition that their fighting men help the other tribes possess the land on the west side. This had now been accomplished and they were headed back home.
When they came to the Jordan, they set up a large altar. This was taken as a sign of rebellion, that they built their own altar instead of recognizing that the one altar should be at the tabernacle. The 9½ tribes felt they had to quell the rebellion.
Fortunately they had a discussion about the matter before they attacked. The 2½ tribes explained that this was not meant to be an expression of rebellion at all. They were concerned that in years to come the 9½ tribes might deny them their part in the altar at the tabernacle. This altar, built on the same design, would prove they should have a part in the worship at the tabernacle.
How often when we see someone do something, we assume they are doing it for certain motives, perhaps the motives we ourselves would have under such circumstances. The result is a misunderstanding. How important to keep lines of communication open to others, especially those in our own family. God wants us to live in peace and harmony with others and that only comes when we communicate openly.