The Bible gives us only one reason why God was angry with Moses. Only one:
And the LORD spake unto Moses, saying, Take the rod, and gather thou the assembly together, thou, and Aaron thy brother, and speak ye unto the rock before their eyes; and it shall give forth his water, and thou shalt bring forth to them water out of the rock: so thou shalt give the congregation and their beasts drink. And Moses took the rod from before the LORD, as he commanded him. And Moses and Aaron gathered the congregation together before the rock, and he said unto them, Hear now, ye rebels; must we fetch you water out of this rock? And Moses lifted up his hand, and with his rod he smote the rock twice: and the water came out abundantly, and the congregation drank, and their beasts also Numbers 20:7-11
God was angry with Moses because Moses did not follow His commandment. Moses was supposed to speak to the rock, not hit it.
If it was any other rock, it would be illogical for the Almighty Father to be angry with him. Alas, the rock was a very, very, very special rock...
Moreover, brethren, I would not that ye should be ignorant, how that all our fathers were under the cloud, and all passed through the sea;
And were all baptized unto Moses in the cloud and in the sea;
And did all eat the same spiritual meat;
And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ. 1 Cor 10:1-4
Moses failed to give the Lord the credit for supplying the water, instead he took credit for the water himself.
12Then the LORD spoke to Moses and Aaron, "Because you did not believe Me, to hallow Me in the eyes of the children of Israel, therefore you shall not bring this assembly into the land which I have given them." Num 20:12 Nkjv
Back when the Lord first called Moses, the 'Rod' became the scepter of God.cf Exo 4:2-5.
When Moses was instructed to take the 'rod', he was to approach the rock as the Lord's appointed delegate. cf Num20:8,9.
Regardless of whether he understood he was to strike the rock or just speak to it, he was the Lord's representative; yet he failed to represent the Lord and give Him the glory, instead he took credit before the children of Israel for the water to himself.
You all have the right answers but we can go a little farther and say that there is a significance between what Moses did.
I also understand the rock to be a type of Christ the true Rock ( 1Cor.10:4), and the water to be a type of the water of the Word through which we are saved and spiritually refreshed
The first striking of the rock took place early in the Exodus and is a picture of Him who was stricken for us (cf. Is.53:4), but the second striking of the rock took place at the end of the 40 years in the wilderness, where the rock was clearly meant to be a symbol of the victory that is based upon our Lord's sacrifice, , just as the entrance into the land is typical of the restoration of Israel
Even Moses' disobedience was used by God to paint a picture of Christ.
We had finished up a service and an 11 year old girl approached me and asked: I have been looking at the two tablets of the law, and you have said that when there are two things, one represents the earthly and the other the heavenly.
Yes. Can you identify the heavenly law and the earthly law?
Yes. The first four and the last six.
But they were broken or split, and you have said that all the things that were split and torn represent Christ on the cross. So do they also represent him?
I don't understand... Jesus was not torn on the cross... Oh wait. The Father was separated from the Son, so God was torn!
While many preachers were focused on the Mel Gibson account of a physical death. She saw the real horror of the cross.
Many of the sensus plenior pictures of the cross are doubled. The water was parted twice, the rock struck twice. This is because Christ was torn twice. The earthly tear was between his body and spirit, the second between the Father and the Son. God had seen many people die. Jesus' physical death was not spectacular. It was a common death on the cross. But when God was torn it angered the Father and Grieved the Holy Ghost. This anger is typified in the anger directed toward Moses. God knew he would strike the rock twice. It did not surprise Him. He expressed the anger at Moses specifically to typify the horror of the heavenly split between Father and Son.