Creepers destroy portal obsidian

So I had a creeper sneak up behing me and go boom while I was next to a nether portal. I had heavy armor so it wasn't a big deal, but when I turned around I noticed the portal was out and only the top 3 and bottom 3 pieces of obsidian remained. The 2 on each side were gone it looked liek

______

______ instead of

_______

_______ not to scale , but you get the gist. I have a screen on my laptop and I can post it later need be.

Anyway I hopped out and back in and the obsidian had reformed, but the portal was still out/destroyed.