When asking most traders this question, the common assumption is ‘a large increase in margin.’
However, the impact of market moves on margin depends upon the type of margin; Initial Margin or Variation Margin. Each are impacted by large market moves differently.
Almost all ETD CCPs use the SPAN (or a SPAN like) methodology for margin calculations. The one exception to this is Eurex that has switched to use a VaR based methodology.