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Java 154 bytes (without using String)

Thanks to manatwork

public static void main(String[] args) {
    for(int i = 0 ,k=1; i>-1; i+=k==1?1:-1){
        for(int c= 65, d = 1; c>64;c+=d==1?1:-1){
            d = d>-1 && c < 65+i?1:-1;
            System.out.print((char)c+(c+d<65?"\n":""));
        }
        k = k>-1 && i < 25?1:-1;
    }
}

Codegolfed

for(int i=0,k=1;i>-1;i+=k==1?1:-1){for(int c=65,d=1;c>64;c+=d==1?1:-1){d=d>-1&&c<65+i?1:-1;System.out.print((char)c+(c+d<65?"\n":""));}k=k>-1&&i<25?1:-1;}