My first, second, and third guess is that he's just very hungry. This is the time of year when BP's really start pounding the food b/c most will stop eating during breeding season. Also, be glad he switched from mice to rats quickly, many "mousers" stay picky and simply won't touch a rat.
You said he got pre-packaged small rats from the store, so I'll assume F/T. If they were the "Arctic Mice" brand then their small rats are pretty danged small, I think 50 grams each is typical. So, I could see an adult male easily downing two of them and wanting more. Look on FB or CL for a local supplier who can sell you F/T mediums, and offer one of those each week.
For handling, I would avoid it for 2 days after feeding, give him time to digest. Otherwise, diving into his hide and hissing is defensive not aggressive behavior, it just means he's happy and comfortable in his "house" and doesn't want to be messed with.
ETA: Do you have a thermostat regulating that UTH?
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