From a report at GeoPandas: geopandas/geopandas#1145

Explanation: assume you have an ExtensionArray with list-like values (eg with an object numpy array with lists under the hood). When calling, to_csv, which goes through Block.to_native_types, we convert this to a numpy array and call astype(str):

But, in numpy, if you have a numpy array with list like values, doing astype(str) does not work:

In [2]: a = np.array([[1, 2], [3]], dtype=object) 

In [3]: a.astype(str)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-db38302b5f9a> in <module>
----> 1 a.astype(str)

ValueError: setting an array element with a sequence

This could be considered a numpy issue, but, we should also avoid converting to a numpy array while we can directly call astype on the ExtensionArray.