The most likely cause of what shows in your photo is overly wet soil either from over-watering, heavy rains, or poorly draining soil mix. Peppers don't like continuously wet roots and leaf margins curl and brown and it sheds leaves directly at the stem junction when kept overly wet.

Plus if it has been especially cool weather lately that makes the problem worse as the soil is still too cool. Usually letting the soil dry substantially and warm up will stop the problem.

Weather this year is creating many unusual problems that only delaying planting beyond the norm will resolve.

Dave