I cannot for the life of me figure out what must've possessed Bungie to make these changes.
Yes, it was understood that Auto Rifles were used far more than Battle and Semi-Auto Rifles. The Battle and Semi Rifles needed a bit of a buff to make them attractive options. But the Auto-Rifles didn't need to be made damn near useless; they can't even perform their intended function - close range multi-target engagements.
And why did the Handcannons need a buff at all? Yes, they were rarely used before but that wasn't because they were underpowered; they were a high-skill weapon. A good Handcannon user was a real threat; a master Handcannon user was an absolute monster to face especially with all the nasty things Handcannons apply. But now they pretty much outshoot anything that can't one-shot them. Whats the deal with that?