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.
You are right that it could be random or some factor having nothing to do with human provocation...
Its just that this pen was 50 years old and apparently for all that time the tigers could have left any time they wanted.
My cat was about 2 feet long and could easily jump on a 6 foot mantle. Using the wall he could get on a 7 foot book case. For a 8-10 foot long tiger, 12 and half feet just isn’t that high.
You should see what my wife acts like when she is in heat (ducking)....