I've noticed that the material used to make the classic index nock that I've used for years has changed. Before, the material was softer and more flexable where as now the nocks are more brittle. They break away easier when hit by another arrow with less damage to the wood taper. That's good and bad. Less damage to the wood taper is good, having to replace a lot of nocks isn't.
I'm sure it's something very simple with your nock but if you want to prevent futher shaft splitting you need to apply a thread wrap right below your arrow nock. After hitting and having to replace hundreds upon hundreds of plastic nocks without ever splitting the first shaft I feel confident in suggesting this. Matter of fact, I won't make any arrow up without doing this. Be it hardwood or soft.
I suspect your nock is splitting from to tight a nock/string fit and the nock material is somewhat brittle. Lot of other factors can place unneeded stress on your arrow nock aswell. Such as, release, improper arrow spine, nock height, double string nocks and the like.
ART B