Relation between poles and cut off frequency of a filter
The Butterworth filter is in fact a continuous-time filter, and what you are seeing is very probably a bilinear transformation applied to this filter in order to get a discrete-time filter.

Software such as Octave or Matlab do this when you ask for a digital Butterworth filter.

The original Butterworth filter, in continuous time, $H(s)$ has all poles equally distributed around a semicircle in the left of the $s$ plane. The radius of this circle is the cutoff frequency (in $rad/s$).

The relationship between frequencies in discrete-time ($\theta$) and continuous-time ($\omega$) given by the bilinear transformation is: $\omega = \frac{2}{T} \tan (\theta/2)$ (where $T$ is an arbitrary sampling period used during design).

You can imagine the left $s$ semiplane as mapped into the unit circle in $z$, or the imaginary axis mapped to the unit circumference.