Comment by jpgvm

Especially when one of the main reasons for needing to post more collateral isn't necessarily that your position is underwater but rather volatility has picked up and risk needs to be brought down. This happens because futures especially trade with very high leverage.

Also generally the amount of collateral that needs to be posted is in the millions of USD so people tend to understand that isn't instant, especially if those funds are currently in money market funds or bonds held elsewhere.