How do acrobatic pilots avoid pulling too many G’s and passing out?
There are certain things you can do to increase your G tolerance without a G suit, mostly static tension exercises like squeezing your diaphragm, lower torso and thigh muscles, isometric exercise style, to slightly slow down the blood flow to your legs during the pull.  Plus just really good physical fitness so your body can handle weighing 4 or 5 times normal for short periods (neck muscles are critical for this, more so if you're wearing a helmet).

Beyond that, and without a G suit, it's just awareness of how hard you can pull and knowing where to ease off.  You have a G meter in the plane to indicate where you are, but if experienced you should be able to tell within reason by the sensation.  

You start to lose your vision (grey out) before you pass out completely, so that is a key warning sign that you'd better back off right now.  However someone who pulls too hard too fast may pass through the grey out period to pass out phase too quickly and it's lights out for a bit.   In the end, the pilot just misjudged and screwed up, probably by getting himself into a situation where the choice was to pull too hard or hit the ground.

Great link provided by a commenter for more info.