It was the wood. Big Box stores sell "construction common" 2X3 and 2X4
stock.  My great uncle complained about the quality of construction
grade wood back in the 50s; said it was "so green you can still hear
the birds tweeting in it."  I wish I could still buy what he hated
because what you get today is much worse.  I've driven screws in and
had them squeeze out sap as liquid as water.  That's what the weight
is in con-common wood.

If Titebond didn't hold it, Elmer's won't either.  I gave up Elmer's in
favor of Titebond almost 30 years ago.  I like the tack better by quite
a bit.  I very occasionally use epoxy, and more often use hide glue,
but Titebond is the glue I use the most.  Titebond II is "water 
resistant,"
and except for outside use, is water proof for all intents.  Titebond 
III
is water proof, period, and I buy it a pint at a time because of shelf 
life
issues.  But it holds like a demon.

The thing is, no glue works well with green wood.  To work, glue has to 
be
able to penetrate the wood. The greener the wood, the less it will do
that.  It's like trying to put water into a wet sponge -- won't happen.

My narrow (30 inches wide) bench is built from clear red fir that I
salvaged from a garage built in 1907.  No muss, no fuss, no gaps.

You are building a bench that will last you the rest of your life. Don't
get cheap construction lumber from any of the big box stores.  Go to an
actual lumber store.  If you can find southern yellow pine, kiln dried,
that will serve you well, and I think you might be able to find it in
NC. If not, vertical grain fir will do.  None of it is cheap, but it's
for the rest of your life.  You only have to buy once.

What probably happened is that the planing you did exposed the wet core
of your 2X4s and allowed rapid drying of that wet core.

Mike in Sacto