This is pure speculation on my part, but here it goes...
Pembroke's motivation for charging ahead can be inferred thusly: he already beat Bruce the year before at
Methven. To quote the
Loudoun Hill Wiki, "Bruce and Valence had first met in combat the previous year at the Battle of Methven just outside Perth, where Bruce's lack of preparedness, and his somewhat conventional military tactics, had brought him to the edge of disaster. His army virtually disintegrated under Valence's rapid onslaught, with many of Bruce's leading supporters falling captive."
If hitting fast and hard works once, it'll work again, right? Perhaps that was what Pembroke was thinking. He figured that if any trouble presented itself, he could just order a charge, and the enemy would break just like at Methven. Pembroke didn't expect Bruce and his men to have learned any lessons from the prior battle. To his misfortune, they did.
There's an old saying about how generals always prepare to fight the last war, not the next one. I'd go with that here: Pembroke was fighting the prior battle, not the next one.
Not sure if that's any help or not, but there it is.