Bidding at the last minute is called sniping on the ebay bidding message board. Go there and you'll find links to several pages that explain why people bid like that. It's mostly to protect them from people jacking up the price.
If you bid your absolute true maximum for the item and someone beats you out, they wanted it more. When you get beat by a small amount, look and see that it is probably one bid increment more than your bid. that's how ebay is designed to work. I snipe all the time, bidding my true maximum and several times I get the item for much less than my maximum bid. Other times I have beat other snipers because I bid my true maximum. I even lose because people wanted to pay more than I did.
True maximum is the most important term on eBay bidding, remember it and your emotions won't control your bidding, which is what you get into when you look for a bargain by "nibbling" at the high bid. Pretty soon you want it so bad your emotions take over.