Hmm, could be 2 things or a combo of bolth.
1st up is low light at plant interior is causing normal leaf die off. (Their shaded by the other leaves)
2nd up is that the soil or your nute mix is higher in P&K then is needed at this point. ( A high P&K early leads the plant into advanced flowering stages that make it use N from the plant more then from the soil or nute feed. Using/adding more N at this point won't work and will just cause an N toxic state.)
Now then, just cut back on any additional P&K you may be using until week 4. At week 6 and 7 is the big bulking stage and a little extra P&K here again can help size and density.
#1 is what it is and it's natural.
Don't sweat it. They'll finish out ok...
Good luck