The trigger for lake overturns in Lake Kivu’s case is unknown but periodic volcanic activity is suspected. The gaseous chemical composition of exploding lakes is unique to each lake; in Lake Kivu’s case, methane and carbon dioxide due to lake water interaction with a volcano. The risk from a possible Lake Kivu overturn would be catastrophic, dwarfing other documented lake overturns at Lakes Nyos and Monoun, since approximately 2 million people live in the lake basin.
The USGS maps show the epicenter on dry land, but with a possible error margin of over 4 miles it could just be under the southern part of the lake. Even in the current location the USGS shake map shows a large area of the lake that could be affected.
The BBC has the first reports of casualties, though thankfully no reports of a lake overturn.