We are always lifting stuff. We're bending forward to get stuff out of the bottom of the refrigerator, we're lifting stuff off the ground, we're picking up after our kids. Pick a day and count how many times you lift stuff and it's in the 30-40 range (I've asked patients to do this and done it myself).
This repetitive bending causes stress on the ligaments of our spine. Over time, this stress can finally reach a breaking point and we develop pain. Often, this breaking point can occur in the morning and thus, we associate our bed, our pillow, or any benign cause as the reason our back "went out."
It wasn't the "I was just putting my shoe on doc!" that made your back go out. It was the repetitive bending that over time caused the back to finally give up when you put your shoe on. When you think about it with a little common sense...is your shoe REALLY THAT heavy and your spine SO WEAK that putting on a shoe would make your back suddenly become damaged? No.
So consider every time to lift stuff to stabilize your abdomen by tightening your abs and maintaining a stable back when you lift even the lightest of things. Over time, your spine will thank you.