cua-auto-tabify-rectangles is a variable defined in `cua-base.el'.
Its value is

If non-nil, automatically tabify after rectangle commands.
This basically means that `tabify' is applied to all lines that
are modified by inserting or deleting a rectangle. If value is
an integer, CUA will look for existing tabs in a region around
the rectangle, and only do the conversion if any tabs are already
present. The number specifies then number of characters before
and after the region marked by the rectangle to search.

You can customize this variable.