Basically there are actors borrowing at the low repo rates and using it on higher rate government backed mortgage products. Banks with money don't want to lend to these actors, hence the repo rate hike.
I'd bet more on the European carry trade, with various financial firms attempt to arbitrage the difference between negative interest rates in Europe and positive rates in the U.S. That creates a natural demand for short-term dollar loans:
https://news.ycombinator.com/item?id=21478251