Daddy leaves for work. Your 2 year old is clearly not feeling well today and just isn't satisfied unless she's on your lap. So you situate yourself on the couch in order to best supervise the other two children while snuggling. Just then, here comes your 10 month old. It's her nap time, so she stands at your feet crying and holding her arms out in that adorably irresistible way. If you move the 2 year old, she's going to lose it, and the 10 month old is currently losing it. So you do the only thing you can - pick up the 10 month old and hoist her up on top of the 2 year old to nurse. And you know, if everyone else is snuggling together, your very snuggly 5 year old will not want to be left out. Here she comes to snuggle up beside you, suck her thumb and play with your neck skin (yep. that's her thing).
That, my friend, is how you end up in a 3-baby pile-up. But trust me, there are far worse places to find yourself ;)