DIAGNOSIS The shelf supports (the little plastic clips or rails they rest on) probably aren't fully engaged, or the weight distribution is off. These are friction-fit shelves, not locked in.
FIX 1) Take all the shelves out. 2) Look at the side walls where the shelves sit. You'll see a series of slots or ridges. Make sure the shelf support clips are clicked into the same level on both sides. Sometimes one side is a notch higher than the other. 3) When you put the shelf back, press down firmly on the back edge first until you hear or feel a solid 'click' on both sides, then lower the front. 4) Load heavier items, like large bottles, towards the back of the shelf, close to the wall. This acts as a counterbalance.
IF THAT DIDN'T WORK If the shelves are still unstable, the plastic support clips themselves might be worn or cracked. Inspect them closely. You can usually contact Manastin customer service to order replacement shelf kits. They're a common part to have available.
HEADS UP Don't overload a single shelf. Spread the weight out. Also, avoid using the very top shelf right under the light/vent for your heaviest items. That area can get warm from the electronics, and the shelf might have less support.