I have seen reports that point to two primary causes;
one, an increase in wild animals, especially deer, near humans, and
two, warmer/shorter winters. You can also track the tick explosion via the westward spread of Lyme disease.
I also think that more people live in forested, or semi forested, non-plowed areas today than fifty years ago, which places more people in tick habitat. Certainly, there is a lot less DDT being sprayed around. I think we tend to forget that most of the US was a malarial area in 1900, and heavy mosquito spraying in the 40-50s helped suppress the mosquitos and malaria. Of course, the DDT did a number on other insects, fish and birds.
We certainly had them when I was growing up on the farm, but then we had lots of deer, too. Due to DDT, I never thought that I would see bluebirds in the wild as a kid. For me, seeing them daily on the ranch is a special gift, and a reminder that things can change in a positive way.
All the best,
Peter