Is This a Tell?

[The Dude]

Live NL, Blinds 2-3. Folded to me on the button with a stack of $280, I raise to $10 (my standard raise when nobody has limped ahead of me) with 6 [img]/images/graemlins/club.gif[/img]6 [img]/images/graemlins/spade.gif[/img]. SB and BB call!

Flop: 6[img]/images/graemlins/heart.gif[/img] 5[img]/images/graemlins/heart.gif[/img] 5[img]/images/graemlins/diamond.gif[/img]
Checked to me, I bet $25 SB calls all-in, BB calls (has about $500 left, and is the my best opponent at the table).

Turn: 2 [img]/images/graemlins/club.gif[/img](6[img]/images/graemlins/heart.gif[/img] 5[img]/images/graemlins/heart.gif[/img] 5[img]/images/graemlins/diamond.gif[/img])
BB checks, I bet $75, BB calls.

River: 9 [img]/images/graemlins/heart.gif[/img] (2[img]/images/graemlins/club.gif[/img] 6[img]/images/graemlins/heart.gif[/img] 5[img]/images/graemlins/heart.gif[/img] 5[img]/images/graemlins/diamond.gif[/img])
BB checks, I bet $75, BB immediately and emphatically pushes his whole rack in. It catches me a bit off guard, so I think for a second. 99? 7[img]/images/graemlins/heart.gif[/img] 8[img]/images/graemlins/heart.gif[/img]?

When somebody IMMEDIATELY (and I mean their chips are going in before your hand is even back from betting) raises, does this always mean a strong hand?

After I think for a couple of seconds, my brain tells me I'm nuts for even considering folding here (I'm getting better than 5-1 on my call). My gut, however, still tells me something's wrong (my instinct tells me I'm throwing away $100 here). The old gut vs. brain debate. What do you think? Result surprised me.