How can evolutionary theory relate to teen pregnancy in our society?

First, evolution occurs as the result of many, minute natural selections that happen over a long period of time (thousands to millions of years), so from an evolutionary standpoint, decades-long trends of sexual behavior are pretty meaningless, particularly because they are unlikely to lead  to any significant, lasting genetic mutations.

That having been said, from the perspective of evolutionary biology, teen pregnancy isn't an anomaly at all. The peak childbearing age for human females begins...

First, evolution occurs as the result of many, minute natural selections that happen over a long period of time (thousands to millions of years), so from an evolutionary standpoint, decades-long trends of sexual behavior are pretty meaningless, particularly because they are unlikely to lead  to any significant, lasting genetic mutations.

That having been said, from the perspective of evolutionary biology, teen pregnancy isn't an anomaly at all. The peak childbearing age for human females begins after menstruation. The biological imperative of all organisms is to procreate, or more specifically, to pass on their genetic material (DNA). The younger a parent is, the healthier their zygotes (sperm or eggs) will be, and the more likely that their offspring will be healthy as well. The older a parent is when he or she has a child, the more likely that child is to have genetic defects.

Over the last few hundred thousand years, most humans of our species began giving birth in their teens. In hunting and gathering societies, where survival depended on speed, strength and stamina, young parents had a better chance of providing for their offspring. Also, the ability to pass on one's genetic material wasn't dependent on having one or two children and ensuring that both lived, but on having many children and hoping that a few of them survived to adulthood so that they could procreate before dying. This is a vital distinction to make because modern society is very different from the harsh realities that our species spent thousands of years adapting to.

While teenagers today may make for poor parents because they are not psychologically or economically prepared for the burdens that come with modern child-rearing, from a purely biological standpoint, they are ideal genetic donors. So it is no coincidence that the sexual libido of teenagers is higher than that of almost any other age group. From a purely biological perspective, they are primed to procreate. 

So instead of asking why teenage pregnancy is so prevalent in our society today, you might ask instead how the enormous changes in our society over the last hundred years have caused teenage pregnancy to go from being the norm to being thought of as a societal ill.