2003 aluminum polarkraft crack in top of transom

any ideas as to why this cracked?boat is rated for a 15hp(have a 9.5 on it)also use a transome saver when trailering.the rivots on each of the end caps have popped out at times(drilled and installed bigger rivots)the top of the transom is kind of rolled(one piece)that forms a rail.used some 3'16-1 inch steel at top of transom and thru bolted it.used jb weld to seal crack.tried to weld with the aluminum repair sticks -the stick would not stick(i know it was hot enough used an oxy acele.torch and slowly heated-transome area-it started to melt transome-made more of a mess-maybe not aluminum?now transome has more strenght (also has new plywood on each side of transom)you guys see any problems with this fix?it is above the water line-thanks for any replies

Well I would throw away those garbage repair sticks and have a real welder do the repairs. Never use dis-similar metals or they will promote corrosion such as steel and aluminum. You can get by using stainless steel with aluminum but eventually it will corrode. When rivets no longer hold then it may be time for stainless steel bolts. JB Weld really does not stop ANY cracking in aluminum but it will seal a small hole.

I would remove the motor, visit an aluminum welder (get the welds done right), install new exterior or marine grade plywood sealed with fiberglass resin (absolutely never use pressure treated wood which reacts with aluminum), use only stainless steel fastener's and do it right.

Your other option is to continue to band aid the repairs and maybe be able to tell your friends how a 9.5 HP vanished when the transom broke and the boat sunk!