Are they otherwise healthy?

One of the main causes for bark splitting can be down to severe winter weather like last years, i have noted this on bay, ceanthus and eucalyptus this year, where most of these plants/trees were severely damaged they seem to recover quite well, only to completely die off, on closer inspection i have noticed many bark splits, so yours could be down to this, but like spritzhenry says whats the health of the bay trees, one possible other cause could be overwatering after very long dry period