How did the 2007 housing bubble affect investors of buy and hold?

[Lucas Mills]

First, I'm confused as to what exactly caused the housing bubble of 2007. It sounds like people were unable to pay their mortgages, thus resulting in foreclosure, but why were they suddenly unable to pay in the first place?

What's the risk of the housing bubble situation to a buy and hold investor? For example, my original impression is that I should be able to ride this out (when it happens again) because I'm still getting rent checks each month. But is this a false assumption? Is it such that whatever caused people to stop being able to pay their mortgages in 2007 would also stop them from being able to pay rent, thus putting me in a difficult position?

Just wondering what considerations I should be making in order to make sure I don't put myself in a bad position as I attempt to begin/grow my portfolio over the next few years. If I had a hundred thousand in reserves or so I would even give it a second thought right now, but I don't. Is it wise to utilize no-to-low money down techniques to acquire multiple properties when not linearly scaling the cash reserves?