How does the quantization error generate noise?
"Noise" in this context refers to anything unwanted added to the signal, it doesn't necessarily mean it is gaussian noise, white noise, or any random well-described process.

In the context of quantization, it is a purely algebraic argument. One can view quantization as the addition of an unwanted signal ("noise") equal to... the difference between the original signal and the quantized signal. Note that this quantification noise is not random, and is correlated with the input signal. For example, if a signal is periodic, the quantization noise introduced when quantizing it will be periodic too.