1899 O Morgan Dollar, Die Cracks.

It's actually relatively common with Morgans. 1899-O is, in my experience, one of the issues which is most easily found with extensive die cracking.

It's just my opinion that the cracking of dies was from a combination of factors.

1st not properly hardened dies where the other shell of the die was hardened but failed to have areas inside receive the same treatment.

2nd Over worked dies where they where used for longer periods between servicing.

3rd improperly prepared planchets. Ones that where not annealed correctly and where harder then normal. That would explain why some coins seem to have weak strikes as well. Anyway harder base metal in a planchet will ware a die faster and cause more stress.

That's just my opinion, so its you think there are other reasons feel free to add them to my list.