Here are a few factors:
- Deer populations are increasing.
- There are no winter killoffs due to warming temperatures.
- Invasive plants like Japanese barberry are becoming extremely common and create microclimates that support mice that ticks live on before attaching to deer.
I believe some tick-borne diseases are becoming more common - I know two people that have had babesiosis, and one person and one dog with anaplasmosis. One of the babesiosis cases was not caught by the hospital in the first go-around and required a blood exchange and almost 2 months in the hospital and rehab.
There is a new invasive insect called the deer ked [0] that crawls like the girl from The Ring and may be a new vector for tick diseases (and they fly!).
Highly recommend checking out the CDC on tick-borne diseases [1], using DEET/permethrin [2] (no permethrin with cats, though), and doing tick checks if you spend time outside.
[0]: https://www.cdc.gov/ticks/tickbornediseases/index.html
[1]: https://en.wikipedia.org/wiki/Lipoptena_cervi
[2]: https://en.wikipedia.org/wiki/Permethrin