Sorry to hear of your misfortune
You have a big overhung load - rough weather
plus sprung load and lateral buffeting shocks will easily add loads 3 or more times the static ones which will be high anyway because of the lever effect. The aft most bolted connection will be under the greatest load. Looking at your pic, this will be 2 to 3 times the total weight of dinghy and equipment
1/4" is way too small especially if 100% threaded- you should use 3/8 bolts at a guess with an unthreaded portion under shear.
Although they are classed as stainless, there are some real rubbish bolts around in some of the marine
stores. Go to one of the engineering fastener suppliers on line and get A286 high tensile corrosion
Over-tightening of bolts is also a problem - download a torque table and use a torque wrench when fitting.
There is a little problem that poor quality stainless is especially subject to called stress corrosion
cracking. Exactly what it says - where the bolt is under stress concentration such as the thread root or under the head
, corrosion rates can be enhanced and tiny sites act as crack generators.
When you assemble make sure you get sealant
under the head as well as everywhere else down the hole.
Lastly fatigue was mentioned - this and/or corrosion was certainly involved. The original bolts may have inadvertently been over tightened, greatly reducing fatigue life.
Load the bolt at only 50% of its maximum rated load - get this right with the torque wrench. Very very roughly the shear load you put on the bolt should only be half of the torqued load
When you look at another bridge examine the quality of the welds onto the mounting pads - if badly done this is another place where cracks will start.
Have great sailing!