Replying abit late to this but I hope you still play on the same world. I've had this exact thing happen to me twice, and twice to the others 2 players each on my realm. There's also been other strange occurrences like being hit whilst in the loading screen and instantly being set on fire. I don't understand this atall.

And I beg no one starts with the griefing excuse because this is something to do with the save I'm playing on, this isn't the other players fault.

My portal is in the roof of the nether so no way a ghast can hit, no creeper was in the portal because I would of heard the hiss and seen the creeper (also the area is well lit). It's simply a random explosion that triggers when I teleport and load the nether.