Our bodies consist of muscles of many types, but breaking it down quite simply, they fall into two categories of what I will call lifters and stabilizers.
We could argue that some are for pressing, some are fast twitch, some slow, etc. Let’s agree that this could go many ways, but for the purpose of this segment, I’m focusing on the muscles of the back used in the weighted squatting and deadlifting. So let’s focus on the muscles in the back along the spine, in particular, the lower back.
From time to time, we get members who complain of a sore back the day after something like deadlifts in the WOD, or DB snatch, or even kettlebell swings, or even rowing. All of these movements recruit muscular exertion that puts the back “in play”.
In this picture of Leeny, notice the maintenance of proper lumbar curve on the rower. Nice…
After any WOD, minor soreness can occur. But excessive soreness in the lower back typically points to poor form. In class, coaches will notice a breakdown in form and make corrections. But when working out at home, there’s no one to see the breakdown and likely, more soreness is being felt.
In layman’s terms, the muscles in the lower back we are focusing on today are primarily to be called upon for stabilization. They are not “lifter muscles”. By this, I mean that the lumbar curve should be maintained from the beginning through the end of movements (like the deadlift, DB Snatch and KB Swing mentioned above). 
When the back is allowed to “Turtle”, it has to be straightened creating a proper lumbar curve at full extension. This straightening departs from the use of lumbar back muscles for stabilization, and calls upon them to be a part of the lift itself. The picture at left is an example of a lost lumbar curve and turtling at the beginning of a deadlift. (Yes, this picture was taken for demonstration purposes. The athlete was not allowed to start the lift from this position.)
This is a great example of “where the sore back comes from”. And beyond a sore back, it can lead to muscle strains that can be almost debilitating for a period of time. As you continue at home workouts until June 15, remain conscious of form, and in my opinion, nowhere is that consciousness more important than in the maintenance of the lumbar curve throughout your workouts.
3-2-1-GO!