Numbers 32:6-17

Now Moses reacts quite angrily. Why would he do that? Why would he act so angrily just because they asked for that land? He accused them of abandoning their brethren as the tribes approached the impending war for the land of Israel.

After they were chewed out by Moses, they change their mind and they back-pedaled and decided “we better go to war with the rest of the Israelites.” Once the two tribes explained that, of course, they meant to settle in the trans-Jordan only after helping fight the war of acquisition, Moses was still not placated. He remained critical of them.

So, what still bothered Moses? They betrayed their true interests when they told Moses that they would build enclosures for their animals and cities for their children before joining the war alongside their brothers. When Moses responded, he reversed the order, pointing out that their priority should be their children and not their wealth.