The majority of that is from countries that gain the most from being in the EU due to receiving more than they put in. Where as there is a large amount of growing resent for the EU in founding countries that contribute the most. Especially France and Spain. And if these countries left, or even Britain, then the EU would start to dissolve as funding dries up.
Well, you say that, but look on page 11 of the same report at the results for the six founding nations. Expressed as net positivity (%positive - %negative, ignoring neutrals), the EU has more people viewing it positively than negatively in all six by a solid margin:
Belgium +22%
France +16%
Germany +28%
Italy +14%
Luxembourg +35%
Netherlands +21%
Spain wasn't a founding member but there the figure is +18%. And in all of these countries except France the number saying they have a positive impression has increased since the last survey.