My own thoughts:

The timing if this is likely the result of a few things.
1) Trump's public "we'll nuke you into the stone ages" rhetoric (vs there just being public statements saying that military options are on the table)
2) Trump's willingness to acknowledge Un at all gives him more credibility as a leader; something he desperately craves. No one else was willing to treat him as such.
3) Knowing that they have the knowledge to make nukes means they're happier to act nicer now...because once you have the knowledge, you can always go back and get it done again later.
4) The Olympics (yes, really)
5) Their primary nuclear test site may be on the brink of collapsing and spewing radiation over the peninsula. (Several experts say this is definitely the status of the site, but at least one has claimed otherwise.)