A question about randomize()

In the following code,  when BCDA.randomize() is excuted, it seems to me methd ADD() is also excuted at same time even though it is not explicitly called. How does randomize() work exactly? Thanks in advance!

//BCD addition testbench
//www.asichowto.com

module BCDAddition_tb();

class BCDAdd;
    rand logic[3:0] BCD1,BCD2;
    logic[7:0] result;
    
    function new();
        BCD1=0;
        BCD2=0;
    endfunction
    
    constraint c
    {
        BCD1
        BCD2
    }

    function void ADD();
        result=BCD1+BCD2;
        if(result>9)
           result=result+6;
    endfunction
endclass



BCDAdd BCDA;
logic[3:0] b1,b2;
logic[7:0] r;

BCDAddition U(b1,b2,r);

initial
begin
    repeat(10)
    begin
        BCDA=new;
        if(BCDA.randomize())
        begin
            b1=BCDA.BCD1;
            b2=BCDA.BCD2;
//            BCDA.ADD();
            if(r!=BCDA.result)
            begin
                $display("Class:  BCD1=%d BCD2=%d result=%d", BCDA.BCD1,BCDA.BCD2,BCDA.result);
                $display("Module: BCD1=%d BCD2=%d result=%d", b1,b2,r);
            end
        end
        #10ns;
    end
end
endmodule