why does it happen vs why it happens

Some examples from the web:

• Why does it happen more in young adults?
• Why does it happen generally while sleeping on one's back?
• If it's random, like you say... then why does it happen to people around me?
• Why does it happen in the US?
• Why does it happen when I am in Busan?
• Why does it happen then when it didn't happen eight years earlier?
• Why does it happen to us, people, to mankind?
• But why does it happen?
• Then why does it happen to me?

Some examples from the web:

• Nobody's sure why it happens or...
• Now, that we know what the problem is and why it happens, let us try and salvage the situation.
• I can only show what happens, why it happens and the consequences.
• I mean, you see why it happens, right?
• Don't know why it happens, best bet is to simply suppress the error using php's directive
• It is interesting to see who and what shaped this and that and why it happens that way.
• How many men died, and how many more will die, for nothing but cruelty or the enjoyment of other men? Why it happens?
• And I know why it happens.
• I said I can't explain why it happens, or how.
• In order to be able to overcome this problem, it is important to start by learning why it happens.
• Any idea why it happens to you more than other people?
• Children wonder why it happens, so it's a great opportunity to use the power of science to show them the separate leaf colors and to explain how the leaves actually change colors.
• Splashing and malfunctions: why it happens With more or less frequent use of the taps, over time it is possible that the aerator or jet breaker (filter) clogs up due to the build-up of solid or solidified deposits left by the water.
• The truth about what happens and the reasons why it happens can be made known.
• Still on the subject of anti-Semitism, "I cannot explain why it happens, but I think that, in general, anti-Semitism is more tied to the right, it finds itself more at home in the political right than in the left, no?".
• This way, for all those who might experience this problem it is not only important to learn how to overcome it but also why it happens in order to be able to prevent it as much as possible as well.
• Why it happens that many serious and traditionalists don't ever recommend not fanatics in certain terms nor would use the same insulting Editor's style.
• That's why it happens.