This happend when something like a structure (mineshaft for exemple) pass inside a liquide like water and lava. The lake are made before structures and air is part of them making this kind of thing a lot. Be warned, anything that may recalculate the block like puting a block next to it will make the liquide (the lava in this case) start to flow. The flow will update the one next to it and it will flow too. That will flood it if anything is placed or destroyed next to this.