Improperly Installed and Sagging Sink
The proper sink installation should include the holes drilling, brackets mounting and sink rim water seal with a silicone. The proper amount of holes and brackets should permanently withhold a sink with full of water or any other normal weight. For a regular square sink the 4 brackets is enough, for the double sink should be installed 6 or more brackets.
Some counter top installation companies do shortcut on the process and adhere a sink on a silicone only, it may hold a few dishes some time, but for long time, or if you have a habit to wash dishes with a sink full of water, it may lead to sink detaching and falling down.