The tiger may have been in heat! Other than that there are 100's hundreds of cases where animals attack for no apparent reason.
Was watching discovery once where these two guys raise a couple of tigers from cubs and took them back to Africa taught them how to hunt. etc. One of them walk in the cage with their two pets one day and didn't realize the female was coming in season and the male attack him when he got between him and the female. Lucky for him there three men there to beat him off.