There were two reasons. First, the e-minis didn't advance above yesterday's 1169.50 close overnight. In fact, they dropped all the way down to 1162 before rallying prior to the pit open. This behavior was a warning sign. If yesterday's high volume breakout above 1165 was the start of a bigger move the market would have immediately moved above 1170 after yesterday's close.
The second reason was the inability of the ES to move back above yesterday's close after today's pit open. The morning rally only made it to 1168.75. After it rolled over I started to suspect that yesterday's breakout was a fakeout so I got out.
The market has dropped to the low of my range estimate for today, 1161. The move down showed pretty good volume relative to recent activity, but nothing spectacular. What really concerns me is the possibility that yesterday's action was a terminal thrust (false breakout) and that a reaction of 20-30 points is underway. So right now I am thinking in terms of 1156 as today's likely low rather than 1161.
My next trade will still be on the long side, probably after a reaction down into the 1150-56 zone.