Cause, potholes, age, wear, vibration, unbalanced tires, off-roading, the tech that touched it last didn't properly torque it, basically anything etc. The point is you caught it before the wheel fell the f off.
Before we go further, allow me to ask a dumb question; are you sure it's your LCA? I ask becasue theres another wibbly wobbly component in that neighborhood you might feel moving if you're going by the old tire wiggle. When were your lower ball joints last changed? Are you running oversized tires lifted etc? Cut the ball joint interval by a third, off road frequently too boot, halve it.
That out of the way, yes it's definitely unsafe to drive, you know the whole wheels fall off thing etc. If the the lower control arm is loose it more than likely needs an alignment. I would take it to a dealer to do so, or in the very least a well reputed shop. I.E. not a jiffy lube type place. Definitely not the last place that aligned it. Try to go mid morning on a tuesday, that way the techs are fresh, have shed hair of the dog from the weekend and aren't in a rush to get home or tired and forgetful from turning wrenches in the heat all day.
(Edit: Unsafe to drive when loose, tighten that som-betch good and snug and drive over to your favored alignment shop. Have the tech do a good torque check on all the hardware down there too.)
After working for a dealer I can tell you the best way to get poor service is to come in last minute on a Friday. Any time Monday ain't much better, there seems to be a common pass time among techs with lingering effects.
