Ouch. I have never seen a bronze coin do that but the photo shows that it had very unusual internal corrosion. Clearly not your fault.
I've smashed a few coins in my time. More often crystallised silver. MY
Republican bronzes seem to be immune to
bronze disease, possibly because of the high lead content (not sure what that does). Or it may be that I rarely buy cleaned bronzes (prefer a little residual crustiness), and I NEVER clean my coins. Someone on this list remarked a while back that rampant
bronze disease almost always has to relate to human intervention (because logically speaking, if the coin
had such a big problem when in the ground 2000 years ago it would have disintegrated 1995 years ago).
One time I bought an evidently fragile but entire crystallised silver coin and received a small bag of dusty fragments. I didn't complain - I got what I knew I was
buying.