Weeping jelly occurs due to

Weeping jelly occurs due to low sugar and adequate pectin as well as because of fermentation.
The phenomenon of spontaneous exudation of fluid from a gel is called syneresis or weeping of jelly.
Weeping jelly is most often caused by too much acid in the fruit or recipe in general. A low pH gel is very brittle and will squeeze water out, producing a liquid layer between the jelly and the glass. Addition of excess acid results in the breakdown of the jelly structure due to the hydrolysis or decomposition of pectin, which helps in the solidification of jelly.