But the bank that has your mortgage on their balance sheet can get margin called. Ask Lehman Brothers how does that happen :)
I heard through a realtor / friend that during '08 there were some home equity loans that were called back though. I don't know the exact details of those who had their HEL's called but my friend stressed that mortgages would never be called back (unless capitalism collapses) but HEL's could be even if you're making on-time payments depending on terms.
It's also not uncommon to see maintenance (ie tested every x month, not just on initial borrowing) loan to value covenants in commercial Real Estate lending, including at quite small scale (think small independent hotels and the like).