Botulinum spores can withstand fairly high temperatures and is an anaerobic bacteria; that makes it hardy against for the two major features of cans that keep the food safe. One of the real dangers of botulinum is that food safety rules that protect against other bacteria are insufficient.

A can doesn't need to be burst or leaking to get the bacteria inside because the spores are probably already there. It's when the can or jar was heat-treated at a temperature which was too low (still hot enough to kill off everything else!) or the conditions inside are not acidic enough that it will germinate from the spores and start producing botulinum toxin.

And, because it's the toxin that causes the disease, not the bacteria, re-cooking the food to temperatures that we would usually consider safe might kill the bacteria but the toxin is still there.