They're seeing just how impotent the new President is. Back in May 2001, the Chinese downed a U.S. spy plane and captured its crew. The ineffectual response from the Bush administration was, I'm sure, watched very closely around the world. Four months later, three jets were used as weapons against the buildings that symbolized American financial and military power.
Someone is watching the Trump administration's reaction to North Korea's provocations, and making a careful assessment.
and he will do it.