How did the bursting of the housing bubble directly lead to foreclosures?

I understand the fact that declining house prices often leads to foreclosure, as it creates a situation in which owed loans are more significant relative to the price of a house.

What I don't understand--and the crux of my question--is how a bursting house bubble gives rise to somewhat immediate foreclosure when:

1. Installments on the house remain the same in price and due date.
2. The salary of the homeowner has not been affected.

Wherein lies the problem if only home VALUE is altered and not AMOUNT OWED per agreement?

**Hoping for answer in basic terms

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