If cold weather had already set in and the slab was thoroughly chilled or frozen, then suddenly turning on the heat could lead to thermal cracking. Barring that, turning on the heat may have just accelerated the drying process and the cracks you mention may have occurred eventually even without the in-floor heat.
The specific crack pattern might give some clues to whether it was normal shrinkage cracking or thermally induced. To avoid problems during future cold-weather start-ups, the slab temperature should be raised gradually over a period of several days.
You don’t say whether these were electric or hydronic installations, but in either case, radiant heating systems only raise the floor temperature into the 80º F range. Attention to details is important in these systems. For example, in-floor installations should have insulation below and around the edges of the slab.
The Radiant Panel Association (www.radiantpanelassociation.org) is one good source of information on this technology. It offers design and installation guides as well as CAD details. Many manufacturers of PEX tubing, a very popular medium for hydronic systems, also offer useful information. You might start with Wirsbo and Watts Radiant.