In response to https://stackoverflow.com/q/52841389/3135417

weird_string_to_datetime.py

# The arbitrary and opinionated conversion of strings to datetime in Series comparisons

# When comparing two values where the left side values is a pandas datetime object,
# Pandas casts the left side values to pd.DatetimeIndex
# Once the casting is successfully, Pandas then calls the __eq__ method on the result of the cast operation.
# In this case, you want to take a look at pd.DatetimeIndex.__eq__ to figure out how the comparison is being handled

# The following is how pd.DatetimeIndex.__eq__ is defined (it's a decorated function, so the function name doesn't come up):

def wrapper(self, other):
    func = getattr(super(DatetimeIndex, self), opname)

    if isinstance(other, (datetime, compat.string_types)):
        if isinstance(other, datetime):
            # GH#18435 strings get a pass from tzawareness compat
            self._assert_tzawareness_compat(other)

        other = _to_m8(other, tz=self.tz)
        result = func(other)
        if isna(other):
            result.fill(nat_result)
    else:
        if isinstance(other, list):
            other = DatetimeIndex(other)
        elif not isinstance(other, (np.datetime64, np.ndarray, Index, ABCSeries)):
            # Following Timestamp convention, __eq__ is all-False
            # and __ne__ is all True, others raise TypeError.
            if opname == '__eq__':
                return np.zeros(shape=self.shape, dtype=bool)
            elif opname == '__ne__':
                return np.ones(shape=self.shape, dtype=bool)
            raise TypeError('%s type object %s' % (type(other), str(other)))
        if is_datetimelike(other):
            self._assert_tzawareness_compat(other)
        result = func(np.asarray(other))
        result = com._values_from_object(result)
        # Make sure to pass an array to result[...]; indexing with
        # Series breaks with older version of numpy
        o_mask = np.array(isna(other))
        if o_mask.any():
            result[o_mask] = nat_result
    if self.hasnans:
        result[self._isnan] = nat_result
    # support of bool dtype indexers
    if is_bool_dtype(result):
        return result
    return Index(result)

# As you can see, the method checks that the right hand side value is either datetime or some form of string.
# If that's the case, then it converts the value to an M8 type (this is a numpy type for datetime objects).
# That is where the conversion occurs.
# The fact that it is not documentated is beyond me, since this is quite an opinionated thing to do.