My guess is that it's getting crowded in there, and some of the bottom leaves are not getting proper light exposure, nothing more. If you can, do things to allow more light penetration, increase the light exposure, or perhaps get more air movement, sometimes that can help too.
With a grow such as this, requires excellent airflow, so you cannot over do that part. If I get dried leaves, I remove them so that you don't get anything growing in there, otherwise, dont' mess with them until their hard/dry and useless, let the plant do it's thing naturally.
Overall, they look pretty healthy, just a little crammed, which isn't a bad thing in itself, as long as it's properly maintained. Good air movement underneath is an excellent idea in this situation, I would suggest. Also, a shot of 1tbsp of "Dr Bonners" peppermint pure castile soap and a tsp of canola/olive oil might help prevent any problems with mold/pest, but I would do it soon, before flowering takes hold.