This is exactly what a hognose does, and any amount of research would have explained this. There are some hogs, no matter how much you handle them, will hiss like their first week in captivity. Some will settle down nice, but even then they will still hiss and flare up frequently.
Why the change? I find myself saying "who knows" a little too often on these forums, but it applies the best. At that young age, neural pathways are still being established. Perhaps there was a subdued behavior associated with a sudden change in environment. Now that is starting to settle in, it is starting to revert to it's natural tendencies.
Since a hognose does not bite its potential predators, it's really not fair to the snake to label it as aggressive. Instead, it is purely defensive behavior. Because they don't bite, no glove is necessary, only nerves of steel to reach in despite elaborate show. If your hognose bites you purely out of defense, and not when it's death feigning, write me immediately because it would be something new to science.
I hope that helps you to look at it differently. Just be courageous and pick up the snake even when it is doing its false striking. Also, if it suddenly stops eating, you need to reduce or eliminate handling since neonates get so stressed out from handling.
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Virginia Herping
http://groups.yahoo.com/group/VaHS
Virginia Herpetological Society
http://fwie.fw.vt.edu/VHS
"The irrational fear of snakes is the only excuse a grown man has... to act like a complete sissy" - Colchicine
... nature has ceased to be what it always had been - what people needed protection from. Now nature - tamed, endangered, mortal - needs to be protected from people. When we are afraid, we shoot. But when we are nostalgic, we take pictures.
Susan Sontag