Acceleration to displacement integration. Displacement plot problems.

H *i,

I'm working on accelerometer data to obtain the displacement. After applying highpass filter I obtained pretty nice looking velocity graph but the displacement seems odd.

Here is my code:

if true
  Fs = 25000; %%Sampling freq
  T = 1/Fs; %%Period
  t = 0:T:9.99996 ; %%time
  L = 250000; %%signal length
  [B,A] = butter(1,12.5/6250,'high'); %%highpass filter

    g = szkolenie_diagnostyczneVIBstandPrzekBadnia(:,1); %%Force of gravity
    a = g*9.81;  %%acceleration
    a_filtr = filter(B,A,a); %%acceleratiof w/o DC

    %%%%%%%PLOTIN' ACELERATION %%%%%%%%%%%%%%%%%%%%%%
    figure(1)
    plot(t,a_filtr);
    title('Acceleration');
    ylabel('Acceleration(m/s^2)');
    xlabel('Time(s)');

    %%%%%%%TRAPEZOIDAL INTEGRATION(VELOCITY)

    v_trapz = cumtrapz(t, a_filtr); %%trapezoidal integration
    figure(3)
    plot(t,v_trapz)
    title('Trap Velocity');
    xlabel('Time(s)');
    ylabel('Velocity(mm/s)');

    %%%%%%%TRAPEZOIDAL INTEGRATION(DISPLACEMENT)

    d_trapz = cumtrapz(t, v_trapz);

    figure(4)
    plot(t,d_trapz)
    title('TRAP DISPLACEMENT');
    xlabel('Time(s)');
    ylabel('Trap Displacement(um)');

end

So after saving my data as "g", and adjusting it as "a" I integrate it and plot obtaining this:

With around 10 different measurements, few are positive, others are negative.

Does anyone know what could be a problem here?

Thanks :)