Some elements that are not noramlly considered highly radioactive can become so through induced radiation were they are made to become unstable eg fireing a nuetron into a nucleus
Hmm well most of the bigger atoms such as thorium,plutonium and uranium are radioactive, this is due the the nuclear binding energies being to weak to hold the larger nucleuses together,
Critical mass is when there is a large enough amount of a subsatnce to self sustain fission eg enough neutrons flying around to continue the fission process
as to how they got to be radioactive well the awnsers are numerous, they can be the products of a radioactive decay themselves or they were fromed like they are