Causes
The bearing inside the assembly may fail from overuse and misuse — or bad design. The Ninja blender is known to form cracks radiating outwards from the blade bearing and will leak.
Some blenders, like the Vitamix, are designed for running at their max speed in order for the cooling fans to properly work. At lower speeds the bearing seal may melt completely or otherwise disintegrate. Water will then infiltrate the bearings during cleaning and wash away grease and cause rust to form. Do not let water sit in the blade assembly, as water will seep into the bearing. Wash and dry after using. Replace the bearings or entire blade assembly if the blade won't spin.
If the lid isn't tight enough, fluid will seep and leak out of the sides of the lid. Tighten the cup more before blending, and know there's help to open a stuck lid.