When I plug my iPhone into my Macbook, it automatically does a hard reset.

[hellomoto4]

Self explanatory title - I plug it in, makes "now charging" sound then hard resets by it self.

Any reasons as to why, and how can I fix this?

Thanks

EDIT: I just noticed that my iPhone said 'iPhone is activated' and then reset. I'm in Australia and don't have to activate...
EDIT: Now it seems to be working, I got the activated message again and now it's syncing.