1. he has a big hand and has missed value and is making up for it
2. he's bluffing

I would then have to look at his river bet timing, his past value-bet sizes, and try to determine if I think he could be betting Jx for nearly full-pot on the end or if I felt he was good enough to turn 8x into a bluff.
and then factor in that we're playing 5NL and #2 is probably a lot less likely imo