thats cuz it was shaken when you picked it upright most likely…
as for which would allow more co2 to escape, that would be upright, gas molecules are far tinier than liquid molecules… provided that the tops were equally tight… upright would have a greater tendency to release more gases therefor losing more pressure… ie going flat…
and newmans post on uncrushed vs crushed = absolutely correct… i didnt read the whole thread… im just agreeing cuz its accurate