What is the likely explanation for low fetal heart rate during pushing?

Head compression. Typically, the fetal heart rate decreases episodically with contractions during the pushing stage of labor as the fetal head is compressed and rapidly descends in the birth canal. Head compression causes a parasympathetic neural discharge that slows the heart rate as long as the intracranial pressure continues. Other causes: cord compression, placental failure can also occur.
Head compression. Expulsive efforts (pushing) increases compression of the fetal head. There are some dangerous patterns of low heart rate that may lead an obstetrician to intervene in this process.