He posted a manifesto to 8chan right before the attack. The pdf is easy to find. Here is the reason he gave for the attack:

In general, I support the Christchurch shooter and his manifesto. This attack is a response to the Hispanic invasion of Texas. They are the instigators, not me. I am simply defending my country from cultural and ethnic replacement brought on by an invasion.

He mentions that reading The Great Replacement directly inspired him to carry out this massacre. He details the weapons and tactics he would use in the manifesto.