An abnormal human baby with 'XXX' sex chromosomes was born due to

An abnormal human baby with 'XXX' sex chromosomes was born due to formation of abnormal ova in the mother. 

This type of syndrome is very rare and occurs due to defects in meiosis I and II during oogenesis in the mother which leads to production of abnormal ova (XX). 

The fusion of XX ovum with X sperm results in ‘XXX’ offspring.