Why do cucurbits produce so much fluid when their stems are cut?
Looking through some papers it is not clear to me that botanists actually know why this should be the case, and I am sure that you are more qualified than me to comment on this Richard! However, there's some discussion of this in Zhang et al. (2012).

These authors argue that the "exudate from curcubits is copious" because of physiological adaptations due to their growth habit - i.e. they are vines and as such often require the structural support of other plants. Vines compensate for a lack of a supporting stem structure by having xylem vessels that are wider than comparable freestanding species  and "high hydraulic conductivity" (which is a measure of how readily water will flow through pores - in the case of curcubits - very readily). Similarly, sieve tubes and sieve pores are very wide compared to species in other families. Since these sieve tubes have such a large cross-section, they do not readily self-seal.


The Origin and Composition of Cucurbit Phloem Exudate
C Zhang, X Yu, BG Ayre, R. Turgeon - Plant Physiology, 2012 - Am Soc Plant Biol. 158, 3


See also:


Why vines have narrow stems: Histological trends in Bauhinia (Fabaceae), Ewers, F., Fisher, J, Oecologia, 1991, 88, 2