Harsh weather stresses them, and they cannot move inside to avoid it. Now is a good time to protect your plants. Do not leave them on the lawn, they can smother it. Rake them and use as mulch in flower beds. Pine needles work best.
Leaves tend to pack and smother roots.
You can still use them, but you may want to compost them first or grind them with a mower. Use a thinner layer than you use with pine straw. You should not be able to see the soil afterwards. If you can see the dirt, the weeds can see the sunlight and will grow. You can also use pine bark nuggets, shredded hardwood mulch or chips.