According to the "Family Guy" creator, he had no idea that his fanbase would react so strongly to the (temporary) death of the beloved pet. From Entertainment Weekly:
“We were all very surprised, in a good way, that people still cared enough about that character to be that angry. We thought it would create a little bit of a stir, but the rage wasn’t something we counted on.”

 “... It reminded people this is still a show where anything that can happen despite the fact it’s been on for a while.” "Family Guy" is currently in its 12th season, and it's safe to say that the stunt definitely spiced things up.
McFarlane did not elaborate on whether his fans' "rage" motivated him to bring the family pet back from the dead, or if it had all been a part of the original master-plan. But he has confirmed that Brian is back for good, saying, “Would I do it again? No. We already did it.”

Be sure to read the entire interview with MacFarlane here.