The Indonesian Meteorology, Climatology, and Geophysical Agency (BMKG) said the wave was not caused by an earthquake, but was possibly the result of volcanic activity at Mount Krakatoa.
According to https://www.rt.com, video filmed at one of the affected beaches and posted on Twitter shows the wave rolling in as beachgoers scramble away from the water.
aSunda Strait separates the islands of Java and Sumatra. Its coast is located about 100km (62 miles) from the Indonesian capital, Jakarta.
The tsunami-like conditions were likely caused by undersea landslides resulting from volcanic activity at Krakatoa Volcano, according to the disaster agency. The volcano, also located in the Sunda Strait, has been spewing ash and periodically erupting for the past several months.