Looks like some kind of seizure.
Not a seizure nor dehydration...... ‘Orthostatic Tremor’.... is the occurrence of a rapid tremor affecting both legs while standing. A tremor is involuntary, rhythmic contractions of various muscles. Orthostatic tremor causes feelings of “vibration”, unsteadiness or imbalance in the legs and other parts of the body.especially while standing..
There are no known cures for orthostatic tremor, but many people are prescribed drugs to ease the symptoms. The most common one being Clonazepam.