When the viscous fluid is dropped at the center, it starts spreading outwards towards the edge of the platform. As the fluid spreads, its distance $r$ from the axis of rotation increases, which increases the total moment of inertia ($I$) of the system (since $I \propto mr^2$). According to the law of conservation of angular momentum ($L = I\omega = \text{constant}$), an increase in the moment of inertia results in a continuous decrease in the angular velocity ($\omega$). When the fluid finally falls off the edge, it carries away its corresponding angular momentum. At this point, the platform continues to rotate with the constant reduced angular velocity it had just before the fluid fell. Therefore, the angular velocity does not increase again, but rather decreases continuously while the fluid is spreading.