#3 Tiredness
Let's not forget something when talking about Greg Hardy's fight. He has never before been in a fight that has lasted more than 1 minute in his professional career, and more than 2 minutes in his entire career.
In fact, Greg Hardy had seemed so good in his previous three fights including the two in Dana White's Tuesday Night Contender Series, by knocking out his opponents, the fans had never seen what sort of stamina he had. The answer was evident 2 minutes into the 1st round of this fight.
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He was breathing heavy and fighting submission attempts from Allen Crowder. He seemed unable to contend with him, and by the time the second round came around, he resembled a steam engine in the way he was letting out huge breaths. His lack of stamina meant that he was extremely tired by the time that Crowder was down on his knees.
People make bad mistakes when they are tired, and perhaps Hardy was no different and ended up hitting him with the knee across the temple in a bad shot thinking that it was an opportunity. Perhaps if he had not been so exhausted, he might have done better.