I think we can say 35-40% of the population really rejected the Constitution, and the 20-15% left voted no for various reasons:
- the text sucks, is too long and bloated, so in doubt, reject
- they wanted to give the political sphere another warning of the general distrust of the population towards them (including people who voted no just to annoy Chirac)
- they fear change

In the part who really rejected the constitution, there are good parts of agricultural related professions, who have more to lose than to gain, since other countries are more competitive, a good part of chauvinists/nationalists, and the good old plain anti-europe-at-all-costs.