Annual palm tree grows back in the middle of summer

We live in Cary NC,zone 7. Last year we purchased two 3 -4 foot palm trees at lowes home center. As expected they died over the winter. We left the planters outside. Last week I noticed some growth from the nub of One of the "dead" trees. Two short green stalks. I transplanted it to a larger pot and it continues to grow. it wasn't supposed to. Would love to know why? Any ideas. The tree was left outside near the front stairs. Fairly well shielded by the stairs and other shrubs..
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