So I was just going to the nether to collect some wither skulls for another wither fight. I reached the fortress and a zombie pigman just started attacking me. I panicked and I went near another zombie pigman, who did not attack me, while the other one kept on doing so. I quickly ran up the flight of stairs in the fortress. After 2 seconds I came down and the same zombie pigman did not attack me. I was playing 1.8.3 but remember the same problem in 1.8.1 as well.

Sorry no screenshots.