gnus-auto-select-part

gnus-auto-select-part is a variable defined in `gnus-art.el'.
Its value is

1

Documentation:
Advance to next MIME part when deleting or stripping parts.

When 0, point will be placed on the same part as before. When
positive (negative), move point forward (backwards) this many
parts. When nil, redisplay article.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.