When his scouting probe left his base he saw your probe coming in from an abnormal angle. There is no reason for your probe to take that scouting path unless you're up to something strange, in response to this he threw down the second gate.
ROFL . he doesnt even look at the probe, look from his player camera. He doesnt look at the probe at all until it reaches his gateway.
Also, if he believed it was a weird angle he would have moved his probe in that direction to confirm it.
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