Without seeing it a good diagnosis is not possible, but the pitting could have been caused by moisture trapped in powder fouling under the lead. It is probably not going to get any worse, although if you are worried, swab the bore with soapy water to neutralize any salts, then dry fully and oil before putting it away.
Once leading starts it wil continue as the lumps will deform the bullet and allow even more gas past the base, blasting the lead vapor onto the bore; plus pieces of lead will be torn off the bullet sides and stick (gall) to the lead already there. Removing all of it, along with any jacketed material which may be beneith it, should go a long way to reducing future leading. If the severe leading returns, then review the loads you are shooting.