According to the second edition of "Clinical Management of Restless Legs Syndrome" by HB Lee, MJ Buchfuhrer, R Allen and WA Hening:

"Because of their OTC availability and widespread use, antihistamines are among the most common to worsen RLS from the patient's perspective. Despite this, there are few reports describing this problem. One study found that diphenhydramine 25mg, given intravenously, severely exacerbated RLS symptoms. Typically, it is the older, sedating antihistamines that cross the blood-brain barrier that tend to worsen RLS."

So it does not appear that any definitive studies of how the first generation anti-histamines trigger RLS. However, if one were to speculate, the interactions between the histamine and the dopamine systems in the brain probably has something to do with it since treatment with dopamine agonists is known to reduce symptoms.

Thanks for you answer

As side effect, the old antihistamines induce parkinsonisms, and broadly speaking, RLS is a kind of parkinsonism.

I am not sure if RLS is a kind of Parkinson's disease. Studies don't suggest that. But thanks for your response

Please take a look at the following PDF attachment.