What is Hillary Hiding?

It is well known that Hillary has yet to release her tax returns, and previously has stated that she would release them upon becoming the Democratic nominee. Given that this is now becoming an issue in the campaign, why does she continue to wait? What magic does "on or around April 15th" hold, thereby preventing an earlier release that could squelch criticism? Why has her campaign become defensive when the issue is mentioned? I have absolutely no idea, but I can speculate.

Given that her returns will show her sources and amount of income, obviously there is something there that she deems advantageous to keep quiet for the time being. Given that she recently loaned her campaign a boatload of money, it would be unsurprising to see a large income. So what else is there? I doubt it is anything too serious, but given her earlier intent to hold off on the release until locking up the nomination, it is likely something that could harm her in the primary race. Perhaps the amount of her income is so obscenely large that it would alienate blue collar voters? Or maybe a source of her past income is an unsavory sweat-shop running corporation, the disclosure of which would drive away a large swath of democrats? If you put your mind to it, you could probably think of a few more, but it really doesn't matter. What does matter is that she is hiding something, and she would rather we not know until it is too late. Frankly, I find it a bit disturbing.