TLDR: Global warming causes a weakened jet stream, which results in less movement of weather patterns. Less movement makes a normal event 'extreme' by turn rains into floods and droughts. Still highly contested. Personally, I think it might just be confirmation bias, as the definition of 'flood' changes a lot due to urban landscape and population density.
they have no idea how the mechanism work, no model actually predicted it until it was discovered and added as a factor to the models themselves etc. etc.