It's weird because it is not clear from the article whether he went into cardiac arrest when they Tasered him, or a few minutes afterward.
Regardless, Taser places stress upon the heart that can kill certain people with little or no warning. Persons with defective heart abnormalities are especially likely to die when they are Tasered... even if they appear to be perfectly healthy.
It is for this reason that Taser should IMO be limited to last-resort usage only... in this case, it appears they tried hard to calm him down before shocking him. This is crucial. Still, it seems more likely that the man perhaps died because of 3 deputies wrestling him to the ground. It can be very hard to breathe when being pinned down by multiple persons and this often results in death, all by itself. My neighbor for example killed an intruder this way, without meaning to kill him.