i noticed as well..
what i think is:
the have reviewers who may be attributed to certain categories. i.e. - some reviewers get anything tagged as food, some get anything tagged as nature, and so forth.. it would explain why certain types come in batches together for me either that, or some reviewers see a batch, and skip over it for something they'd rather review instead. which is another way that the chronological submit order would get mixed around.
realistically, I think its smart to have certain reviewers for certain categories, if that is in fact what they are doing.