Why is my hair wavy?

I have really straight hair.   Well, I should say that most of my life, I have had really straight hair.
When I was little, my mother kept wanting me to get permanents, which was a constant topic of contention between us.  At some point, that went away.

I just wore my hair straight.  And for years, long.  But it was not thick, and frequently looked stringy...I rarely used cream rinse, just shampoo.  At age 19, I had my first grey hairs.  A friend pulled one out.  I told her to stop, because I would be bald before she was done...And, I did not put any other chemicals in it: no dyes, no tints, nothing.  A hairdresser once told me I did not just have grey hair, but blond and grey mixed with brown.  There was a time when I thought my hair was grey, but others thought I was blond.  And people asked me how I frosted it the way I did.

In the early 90's I had my hair cut in a blunt cut.  It looked thicker, and was much easier to manage.

Sometime in late 2007 or early 2008, I noticed that the hair on the back of my head was starting to wave.  I could feel it, the way it hung.  It drove me nuts.  I could not see it, but I could tell it was waving.  I changed my haircut, to a slight layering in the back, and tapered cut in the front/sides.

That helped for a while.  Now, recently, the sides are starting to wave.  Depending on how it curves, the waves look really good, or my hair sticks out really awkwardly. 

I do not get it. Why after all these years is my hair waving?