Global warming just like in 1868, 1901, 1917, 1923, 1942, 1952, 1972, and 2016.
The 1868 flood was definitely caused by lack of anti-pollution devices on the cars back then because it wasn't regulated.
As we are advancing further into the Grand Solar Minimum (sun is ready to hibernate) I have a feeling we will be seeing more of the same and not just in Ellicott City, along with earthquakes etc. You may want to watch John L. Casey discussing The Grand Solar Minimum earthquakes & more GSM exclusive on YouTube.