Such thing as too much air?

[island]

Last night we opened and decantered a bottle of 2001 Terra Rosa - Cab, which I found out about a couple of years ago. After keeping it in the decanter for about 30 mins or so, the first couple of glasses were great. We did notice however that the last 2 glasses in the decanter and the left over in our glasses (after sitting for about 10-15 mins) tasted a bit stronger, almost like if we didn't decant it at all.
Is this pretty normal? I have had the same experience with the Rodney Strong cab as well.

Thanks in advance.