Why drift is coming in waveform?

The PASCO wireless accelerometer is fixed to the uniaxial exciter. It is made to oscillate at a frequency of 19Hz. It is made to oscillate at constant acceleration using the signal generator

The recorded acceleration plot is sinusoidal approximately.This acceleration data is integrated using the cumtrapz command in Matlab to get velocity. The obtained velocity plot is increasing trend. The exciter is actually oscillating at constant velocity but the velocity plot obtained on integration is increasing trend. Why the increasing trend coming while using cumtrapz for integration to find velocity. Is this approach correct?

The images are in the attachment.

Kindly clarify the doubt.