Why is Python class variable acting like a static variable?

I created a node class like this:

  def __init__(self, data, incoming = [], outgoing = []):
    self.data = data
    self.incoming = incoming
    self.outgoing = outgoing

  def addIncoming(self, incoming):
    self.incoming.append(incoming)

  def addOutgoing(self, outgoing):
    self.outgoing.append(outgoing)

where every instance was initialized like Node(number) so the incoming and outgoing parameters were defaulted to an empty list. I also had functions that appended other nodes to the incoming and outgoing list but when I did that, it was appending to every instance. For example lets say I had a=Node(1) and b=Node(2). If I did a.addIncoming(b) and printed a.incoming and b.incoming, they both would print Node b. Why are these values shared between separate instances?

When I updated my init function to be

  def __init__(self, data, incoming = [], outgoing = []):
    self.data = data
    self.incoming = []
    self.outgoing = []

everything worked as expected but I feel like the two should work the same. What am I misunderstanding here?