please explain!

[hitinvo]

Every once in a while there comes a time in a fellow's life when it seems like his luck had gotten stuck in quicksand and is sinking fast. Just when he thinks it's about to go under, though, he finds a rope and pulls it out again. Now, that's just how things were last spring when my knob-headed old mule and I were roaming the range up near Cheyenne (the excerpt from the Old Coot and the Augering Match)
Question: Could anyone explain this paragraph for me?
What does the 1st sentence mean? Thanks.

[hitinvo]

Does this paragraph use analogy? and what does the phase every once in a while mean?