It could be caused by several things. In order, I would suspect:

- metabolism (especially if it is fairly light syrup)
- very slight bud (especially if it is fairly dark syrup)
- late-season air injected syrup
- off-flavor caused by storage container
- something else

Find someone near you who is really good at determining flavor/off-flavor issues (maybe someone who buys a lot of syrup or who has gone to the IMSI Grading School). If all else fails, send some to me and we'll give it a taste. Any other info you have on the syrup might be helpful as well.