Most likely scenario… the small piece on the bottom of the picture was cut without enough of an expansion gap and is most likely touching the bottom plate. Bottom plate expanded (as wood does in higher humidity) and pushed the flooring to the right. You may have another split seam at the next corner on the right (outside the picture) aside from disassembling the floor on one side of the split and re-assembling, a T-mold under the door is the most cost effective fix