How Common is It for a Molar to Break (Shatter) After Replacing a Filling?

It's not unusual at all for a molar with large fillings to fracture, sometimes repeatedly. Long term, a crown is the stronger, longer lasting choice in a situation like yours. People often choose to have a filling done instead because it's less expensive and quicker to do. Fractures or cracks can be difficult or even impossible to see on an x-ray, since they can be blocked by old metal fillings or simply in the same plane as the image.

You preface your comment about your tooth saying the filling was old and damaged. Often times, fractures will not show up in an xray and most patients would rather pay for a filling than a crown.  Your doctor probably should have discussed a crown before placing the filling, if there was a likelihood of it failing. He is trying to do the right thing for you financially but, sometimes, it works out for the better in the long run to just say you need a crown because the tooth's structure is so compromised. The fractrure probably was not visible in the xrays .

Hi, thanks for your post, and sorry to hear about your tooth, that's very frustrating.  With very large fillings, "full-coverage" restorations such as crowns or onlays are often recommended because once the existing filling/"bad" surrounding tooth structure is removed, the area that remains can be very large, wide, and fragile.  Placing another filling in the tooth does not provide the proper structure needed to withstand the forces of chewing/your teeth coming together (which is amazingly strong), which can cause the tooth to crack.