Answer:
An external force acting on the object.
Step-by-step explanation:
According to Newton's first law, objects remain in uniform motion in a straight line, or rest, unless acted upon by an external force.
An object slowing down experiences deceleration (or, negative acceleration). Acceleration (positive or negative) implies action of an external force.
One example of an external force causing an object to slow down can be a force of friction, however, there can be many other sources of force achieving the same effect (including simply your hand pushing against it).