Why is an API extension treated as breaking?

[RusKnyaz]

I compiled two versions of assembly:

• 1st contains empty class
• 2nd contains the same class and new enum

The report is:

<Assembly Name="test, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" DiffType="Modified">
  <Module Name="test.dll" DiffType="Modified">
    <Type Name="test.Types" DiffType="New" />
  </Module>
</Assembly>[diff] Suggested version change: Major

Why 'Suggested version change' is 'Major' ?
According to semver we have to change Minor version part in cases of API extension.