Reason:
1) Sometimes, when sellers see no bids on their items but a lot of watchers, they end it prematurely to avoid selling low. Some bidders avoid this by bidding early.
2) Not all bidders can monitor their bidding activities. Some just put their maximum bid and leave it there, hoping they win.
3) Even for those bidders who monitor their activities, there might be some activities that will demand them to put off ebay for a while (1 or 2 days); hence, they put their maximum bid early
But yeah, I hate it when a bidding frenzy starts on my watched items.