I remember one of our teachers had told us the adverb 'sometimes' is different (among adv.s of frequency) It appears at the beginning of your sentence... However I don't recall the exact rule.
example: Sometimes I can work ten hours a day.
Some other adverbs require inversion when placed at the start:
Never have I seen anyone do that before.
Rarely has anyone eaten that much.
Sometimes doesn't change the word order. Could the rule have something to do with this?