Anyone know why this started happening in April? I thought the difficulty adjustments are supposed to keep it at 10?
Most probably the difficulty retarget wasn't great enough compared to the rise in hashing power. The difficult is designed to change every now and then to try and ensure that blocks are found every 10 minutes or so, but if there is an under-prediction then it may not be so. Also, with anything luck based there's always going to some variance which leads to a deviation from the expected value.
I really wish that image worked. The image shows that it is not a matter of variance, and you can see the averages clearly separated. If it was variance, it would look a lot more chaotic than this. We have also been through many difficulty changes, and many chances to get back up to 10 mins.
It's a very clear drop, and have not yet come across a good explanation..