Honduras on their own would not have invaded Nicaragua. As or US support Reagan apparently considered the possibility in the early 1980s but it never came to anything.
As for a casus belli it would have to be something big such as s coounter attack to a Nicaraguan invasion of Honduras, a major deployment by Cuba/Soviet Union threatening the Panama Canal. None of which was very likely as it would have been too risky, provoking at least a major crisis. During the Cold War th Superpowers tried to follow the rules in order to avoid an action that provoked the other to that degree and that included the behaviour of allies and client states. Given the proximity to the US it was unlikely the Soviets could win. And the US knew the conflict in Central America was not really a vital strategic issue as long as it stayed limited. Which might be why the conflict never escalated to more than local insurgency conflicts.