It is thought that these are caused by the cervix overriding the front fontanel of the baby's head. This causes an altered cerebral blood flow, with a subsequent slowing of the FHR due to what is called the Vagal reflex. It usually occurs between 4 to 6 cm of dilation, and is not an indication of a decrease in oxygen.
Obstetrics: Normal and Problem Pregnancies, 5th ed. Gabbe et al.
Fanaroff and Martin's Neonatal-Perinatal Medicine, 8th ed. Martin et al.