Tendency of a roller to throw off small droplets of paint during application.
Poor application/rolling technique; applying paint too rapidly
Use of a low quality roller or incorrect roller cover
Use of lower grades paints
Higher quality paints are formulated to minimise spattering. Using high quality rollers, with the appropriate nap length, can help. Overloading the roller with paint will result in excess spatter, as will overworking the paint once it is applied to a substrate.
Working in three-feet square sections, applying the paint in a zigzag 'M' or 'W' pattern, and then filling in the pattern, will lessen the likelihood of spattering.