Why is Technetium unstable?
If you google "why is technetium unstable" you'll see that this question has been around for a while.  Over on physics.stackexchange, it is discussed in Why is technetium unstable?.  

I've read some of what others have posted on this topic and the short answer appears to be: because we have such a poor understanding of the atomic nucleus, there is no answer to this question at this time.  Again, the link above would be a good place to start.  One of the answers (3 upvotes) claims to be an answer.  The selected answer (10 upvotes) says there is no answer.