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Compost

Pretty normal for lower leaves to yellow and drop. If it isn't affecting the upper part of the plant, I wouldn't worry too much. The moisture issue concerns me a bit though. Try to let your plants dry out almost all the way before watering. If your soil is holding too much moisture, you may want to change them to something that drains better.
Bruce

Try hitting them with some nutrients next time you need to water. Like most things, when something unexpected happens, look at what changed recently. In this case, the medium. It probably holds a lot more water than the old mix, plus, the roots haven't filled it out yet, so it probably won't need watering as often for a while. Also, if the compost is still breaking down, it could be using up the available N. So, I would be careful of them staying too wet, and see if fertilizing helps.

I would also second"
-- lower leaves yellowing, falling off is normal. More so in container.
-- General pale green is caused by : too much moisture AND/OR too little nutrients, especially nitrogen. Keep in mind different peppers have different foliage in terms of darkness of green.

--- Unfinished compost can absorb an lock up some of the nutrients for a while until it becomes saturated.

Thanks for the replies.

I've been growing peppers in containers for ten years and I can't agree that yellowing and dropping off of leaves is normal. I've not found that to be the case, certainly not to this extent, at this early stage.

The replies lead me to believe it's related to the compost mix, and it may be a combination of water retention and unavailability of nutrients.

I'm trying a foliar spray to get nutrients to the plants without further dampening the roots. I will also try some Epsom salts.

Hopefully I can save what was until recently looking like a wonderful crop of plants.