Coming a month after June 's Singapore encounter with Kim Jong Un were three back-to-back July summits.

这是倒装句，表语前置，主语后移（完全倒装句，注意主谓一致）。

Three back-to-back July summits were coming a month after June 's Singapore encounter with Kim Jong Un.

这种结构是早期英语的残留形式，即现在分词短语（早期为形容词）作表语。现在把现在分词作表语一律分析为进行时态。