if sexual orientation isn't a choice then why do some straight people turn gay and some gay people turn straight?

It's clear.  Often gay people hide their gayness by going through the motions and behaviors of straight people.  But then, as they mature, they give in to their true nature and "come out."

I've seen happily married couples with kids where one of the partners eventually comes out and leaves the marriage.  The latest was where the wife left her husband to take up with another woman.  As a couple they had three kids and the wife bore all three of them.

But she was gay.

What is called "turning" is actually a misnomer. Turning is not what's taking place. Actually the person is just trying to be one way which they aren't. Because they're really opposite. Finally they just realise it and give up which is what people mistakenly call "turning" but it isn't. But now, that said, some people will always be bi and can do either.

i was straight then i turned gay and i can tell you this, you like apple but you don't like orange, neither by your choice, you just find yourself this way

Bisexuality is a thing 

Straight people don't "turn gay"; the individual was never straight to begin with and needed time to accept oneself as homosexual and come out to others as gay.  Likewise, gay people don't "turn straight"; the individual may have thought he/she was gay, but in time discovered there really was no attraction for the same sex.  That one is a bit less common since there is so much social and familial rejection of homosexuality while heterosexuality has never experienced such rejection, resulting in people who are far more likely to think or pretend they are straight when they are in fact gay (or bi) than people who think or pretend they are gay when they are in fact straight.

Straight people don’t become gay. Sometimes it takes a while to discover who you are. The only reason I can think of for gay people “turning straight” is conversion classes. If this is what you’re thinking, know that these people are still gay. Many choose to hide it now for safety. They obviously weren’t accepted the first time they came out, so they’re covering it up so they’re not hurt. You could also be talking about bisexual people (or any other sexuality) they like both men and women. It doesn’t change depending on who they’re with. 

Very few gays become straight because the lifestyle is addictive. Straights become gay because they decide to experiment and get hooked on gay sex. 

Answer:  

Because for any rule made by nature,  nature will then break it at her leisure.

Additional thinking: 

Only a bisexual person would assume that sexuality is a choice.  They would because unlike heterosexuals & homosexuals,  bisexuals to have the luxury of choice and so also assume because they can make a choice others must be the same.   The flaw in this logic of choice is,  they do not choose to be bisexual. 

They are called Bisexuals 

They don't 'turn gay' or 'turn straight', they simply realize what it is they've always been and didn't know.