In chemistry, generally most of the notable exceptions are associated with the thermodynamics or the kinetics of the reaction.
In the particular example of chloral hydrate, chlorine tends to form hydrogen bonds with hydrogen (even though usually their hydrogen bonds don't exist) majorly due to the thermodynamic changes that occur in the process. By forming such "pseudo-rings" (because H-Bonds aren't completely a "bond", rather only very strong attraction) the structure of chloral hydrate gains excessive stability as 5 & 6 membered rings are very stable. This pseudo-ring formation is called "Chelation Effect".
So, basically that extra stability is the driving force for chlorine to form H-Bonds and the process, as a whole, becomes thermodynamically feasible (spontaneous).