I don't know if it causes fatigue and dizziness in some people, but it is not well understood. Wikipedia seems to say it does several different things, but it sounds like speculation to me.
The most interesting thing in the article is the explanation of why it takes a couple of weeks for SSRIs to become effective. Downregulation is a form of negative feedback, which the brain uses to try to stay in balance when there is an unusually high, or low, level of a neurotransmitter.
Over a period of 1–2 weeks, the 5-HT2C receptor begins to downregulate
, along with the downregulation of 5-HT2A, 5-HT1A, and other serotonin receptors. This downregulation parallels the onset of the clinical benefits of SSRIs.
Receptors are created, or expressed, from instructions in the DNA of the cell, and they can be increased, or upregulated, when the signal is weak, or decreased, or downregulated, when it is strong.