A "premature" failure, as you seem to have experienced, usually results from circumstances not in accordance with the design intent. Fatigue cracks result from what is known as "cumulative damage". In the normal range of loading, this cumulative damage takes a long time to generate a macroscopic crack. If, for instance, the frame had been subjected to many cycles of abnormal loading prior to your acquisition, a large amount of cumulative damage may have been pre-existing.
Another possibility is a quality defect or perhaps an artifact of accidental damage during maintenance, resulting in a stress riser (notch, scratch, or nick in the material) in a highly stressed area, in other words a ready-made crack initiation site, which equates to a large amount of cumulative damage. Since your frame has two cracks, it would be quite a concidence for this to be the root cause in both cases.
It also occurs to me that simple overtightening of the threads (at least in the case of the bottom bracket) can cause an immediate or incipient crack.
On the other hand, some metal alloys are known to go through a brittle transformation below a certain temperature. This is certainly true of some grades of stainless steel, but I don't know if any aluminum alloys/heat treatments are subject to this phenomenon. Again, there could be a quality issue, not in terms of a physical defect, but rather alloy composition or heat treatment (though both of these seem quite unlikely to me). You might check with the manufacturer to find out if there are any known issues with the frame or its material, and whether or not the material has a brittle transformation temperature in the range you have ridden it.