Acceptable variation in accuracy of each k fold when using K-Fold Cross Validation?
It's indeed a quite large variation, but nothing alarming since 9 out of 10 folds are within the range 0.8 to 0.9. 

There are many possible factors: yes a fold can be easier than another one by chance, but it also indicates that the training process is a bit unstable. Increasing the number of instances and/or reducing the number of features often helps reducing the variation.