Is margin variable according to price, and why?

TWDsje 

... However, the exchange decided to increase the margins, and that meant some traders had to liquidate their position resulting in an ugly selloff.

Exactly. Insofar the alert trader can easily understand why nerves were raw in the NQ lately:
Margin debt at all-time highs, a spike of the VXN - which logically implies higher margins per contract.
Although the NQ only lost peanuts compared with its run YTD, it was already hanging by a thread on
the second day of the spike.