Delayed fermentation with wort in sealed keg

[brew_cat]

Pasteurizing juice

Now an interesting thing happened. I have apples that come in at different times so i pasteurized the juice and store it in a keg with some pressure. Today i went to open the keg and it had built up alot of pressure and smelled of fermentation. Odd right. I figured must have been some leftover yeast in the keg although i washed