Why does state instantly go to last state whenever pushbutton is pressed

Question

I am implementing a door lock that requires user password. I have implemented states to identify predefined passwords of 123, 122, and 121, and it unlocks the door after the correct password is entered sequentially. I am implementing this door lock using four push buttons (KEY0 as digit '1', KEY1 as digit '2', KEY2 as digit '3', and KEY3 as digit '4') and a reset, RST (assigned to SW0) on DE1-SoC FPGA board. The link of user manual is below: http://www.ee.ic.ac.uk/pcheung/teaching/ee2_digital/de1-soc_user_manual.pdf

Please refer to the attached image for the states diagram
.

I am facing a problem in which whenever KEY0 push button on the board (at S0 initially) is pressed, it would instantly go to S8(displayed on the seven segment). Is there something wrong with my code? Do note that the push button on DE1-SoC Board is active low.

HEX0 and HEX1 are actually assigned to two seven-segment displays on the board.

Below is my Verilog code:

    module rewardsys(KEY0,KEY1,KEY2,KEY3,HEX0,HEX1,RST);
    input KEY0,KEY1,KEY2,KEY3,RST;
    output reg [6:0] HEX0,HEX1;
    parameter s0=4'b0000;
    parameter s1=4'b0001;
    parameter s2=4'b0010;
    parameter s3=4'b0011;
    parameter s4=4'b0100;
    parameter s5=4'b0101;
    parameter s6=4'b0110;
    parameter s7=4'b0111;
    parameter s8=4'b1000;

    reg[3:0] cstate,nstate;

    always@(posedge KEY0,posedge KEY1,posedge KEY2,posedge KEY3,negedge RST) begin
      if(RST==0)
         cstate<=s0;
      else
         cstate<=nstate;
    end

    always @ (posedge KEY0,posedge KEY1, posedge KEY2, posedge KEY3) begin
      case(cstate)
         s0:
            begin                     //display S0
              HEX1=7'b0010010;
              HEX0=7'b1000000;
              if(KEY0==0)
                 nstate=s1;
              else if ((KEY1==0) | (KEY2==0) | (KEY3==0))
                 nstate=s6;
              else
                 nstate=s0;
            end

         s1:
            begin                     //display S1
              HEX1=7'b0010010;
              HEX0=7'b1111001;
              if(KEY1==0)
                 nstate=s2;
              else if ((KEY0==0) | (KEY2==0) | (KEY3==0))
                 nstate=s7;
              else
                 nstate=s1;
            end

         s2:
            begin                     //display S2
              HEX1=7'b0010010;
              HEX0=7'b0100100;            
              if(KEY2==0)
                 nstate=s3;
              else if(KEY1==0)
                 nstate=s4;
              else if(KEY0==0)
                 nstate=s5;
              else if(KEY3==0)
                 nstate=s8;
              else
                 nstate=s2;
            end

         s3:
            begin                     //display S3
              HEX1=7'b0010010;
              HEX0=7'b0110000;
            end

         s4:
            begin                     // display S4
              HEX1=7'b0010010;
              HEX0=7'b0011001;
            end

         s5:
            begin                     //display S5
              HEX1=7'b0010010;
              HEX0=7'b0010010;
            end

         s6:
            begin                     //display S6
              HEX1=7'b0010010;
              HEX0=7'b0000010;
              if ((KEY0==0) | (KEY1==0) | (KEY2==0) | (KEY3==0))
                 nstate=s7;
              else
                 nstate=s6;
            end

         s7:
            begin                     //display S7
              HEX1=7'b0010010;
              HEX0=7'b1111000;
              if ((KEY0==0) | (KEY1==0) | (KEY2==0) | (KEY3==0))
                 nstate=s8;
              else
                 nstate=s7;
            end

         s8:
            begin                     //display S8
              HEX1=7'b0010010;
              HEX0=7'b0000000;
              nstate=s8;
            end
            
         default:
            nstate=s0;
      endcase
    end
    endmodule

Thank you in advance.