Remember when Fedor fearlessly went into Big Nogs guard? General consensus was that willingly going to Big Nogs guard was stupid. Many years later, staying in Werdums guard was his downfall.
Fedor is a sambo and judo expert and not a pure striker of course, but still.
Some guys are well rounded or think of themselves as well rounded, strikes on the ground change the ground game also. Even in training with softer strikes it can mess with pure grapplers a lot.
I could see in Perrys case it is either lack of fight IQ, or not being able to handle the adrenaline while fighting(which could be seen as bad fight IQ I guess). So he gets overly aggressive and makes careless decisions.
Could be nerves also. When playing up variations of the fight in your head leading up to the fight, the ground might be what you are most worried about, while you also have trained a lot to improve your ground game, training for specific situations for this particular fight. Cage door closes and the adrenaline sets in, you might even take it to the ground to shut up the nerves and in a way rush the scenario you've trained for and worried about...