Steve, basically it was a stability (or instability) issue. The ship was built very high and narrow with insufficient ballast and too much weight above water in terms or armament and decoration etc. This was identified before she set sail but the king expected to see her sail out of harbour so everyone crossed their fingers and hoped. The wind simply blew her over!
Extract from the Vasa website
http://www.vasamuseet.se/sitecore/content/Vasamuseet/InEnglish/History.aspx below:
Why did Vasa sink?In the 17th century there were no scientific methods of calculating a
ship's stability. It was not uncommon that warships heeled over and
sank. Their cargo - the guns - were placed relatively high up in the
ship, whereas merchant-vessels stored their cargo in the hold, ie in
the bottom of the ship.
Instead of using calculations, the 17th century shipbuilders used so
called reckonings, which recorded certain ship-measurements.
However, the reckonings used in building the Vasa were intended for
smaller ships with only one gundeck. The Vasa was built differently.
She had two gundecks with heavy artillery (when the norm was to
place lighter guns on the upper gundeck). The standard rules
obviously did not apply here.
Deep down in the Vasa several tons of stone were stored as ballast.
They were meant to give the ship stability. However, the main reason
for the Vasa capsizing was that the ballast was not enough as
counterweight to the guns, the upper hull, masts and sails of the ship.
In the inquiries after the Vasa disaster it was revealed that a stability
test had been performed prior to the maiden voyage. Thirty men had
run back and forth across the Vasa's deck when she was moored at
the quay. The men had to stop after three runs, well before the test
could be completed - otherwise, the ship would have capsized. Present
was Admiral Klas Fleming, one of the most influential men in the
Navy. His only comment to the failed stability test was "If only His
Majesty were at home!" After that he let the Vasa make her maiden
voyage.
Who, then, were to blame for the disaster?
Admiral Fleming. Partly. He could have stopped the ship after the
stability test. On the other hand, the ship was already complete and
the king was waiting impatiently in Polish Prussia.
King Gustavus Adolphus. Partly. He was anxious to acquire a ship with
as many heavy guns as possible. He had also approved the Vasa's
dimensions and was keen to have her completed rapidly.
The shipbuilder Henrik Hybertsson. Partly. Although he built the hull
too narrow, he was a skilled shipbuilder who had previously built many
good ships. His unexpected death the previous year just complicated
matters.
The captain Söfring Hansson. According to a new theory the capsizing
of the Vasa may be blamed on the captain. He sailed a brand new
ship with open gunports. The Vasa sank when water gushed in through
the lower gunports! It would have been wiser to test the new ship on
her maiden voyage with closed gunports.
However, the inquiries showed that no one could really be blamed for
the disaster. The main reason being the insufficient theoretical know
how of the period. The Vasa was something new - a military
experiment. After the Vasa, many successful ships were built with two,
three and even four gundecks. The shipbuilders learned from their
mistakes with the Vasa and improved later designs.
