The noise in that graph looks like simple Additive White Gaussian Noise (AWGN). The source of such noise comes from various sources, including imperfections in the real RF components, the weather, etc. Because the sources are small, numerous, and unpredictable, the noise is just modeled as having a distribution of a Gaussian random variable around the constellation point. These small points in the cluster indicate the received signal after going through the noisy channel.
Oftentimes the noise is relatively constant in the environment, but the received signal power can vary (depending on the location of the transmitter). The relative difference between the received signal power can lead to a significant increase in the noise you see in the constellation (low received signal power will give a larger cluster around the constellation point).