The most likely thing that will put a lot of stress on the glass is if it doen not quite line up with the apperture (hole) it fits in. This would also be consistent with one of the screens leaking. One possible cause for this is if the car is slightly twisted after a previous accident repair. If you are worried, get a full body alignmement check done (they normaly measure the distance between diagonaly opposite key points, such as suspension mounts).
However, dont panic. I consider it far more likely that your faveroute glass replacement specialist has had its own (cheaper) batch of screens made, rather than buying them off Ford, and they are not quite dimensionaly correct.
When the bloke comes to fit the screen, ask if he has to do a lot of S Max screens recently. Ask whether it is a genuine ford screen (I would expect it to have a small ford logo on it, or words to the effect, "made by XXX for Ford", which he may or may not be able to point out to you).
If the original replacement was done on your insurance, and the next new screen fails, I would contact your insurers and explain that as (by then) you have been through four screens, then either the fitters are incompetent, or the product is sub standard. EIther way, would they (your insurers) be prepared to pay for the next replacement to be supplied by and fitted at a Ford main dealer body shop.
Another reason for the screen cracking, is if the edge of it is chipped before or during fitting. You may not be able to see this after fitting as the edge of the glass will be out of sight. Watch the bloke fit it, and while the screen is out, have a really good look at the paint where the screen goes in. If this is damaged, it may indicate poor previous fitting.
