I have created a simple DOP network to use gravity to make Boxes copied to the vertices of a Grid with a Copy node to fall. I have the copy node's Pack Geometry Before Copying checked to use packed primitives and have used an RBD Packed Object node to point to it in the DOP Network.

The sim works as expected.

However, if I connect an Attribute Wrangle node under the Grid node and halve the size of the boxes with this expression:
@pscale = 0.5;

then the boxes fly apart as if they are intersecting when I run the sim.

What could be the problem?

There is another attribute you can set, if you expect initial RBD intersections. Check out Matt's solution, here.
http://www.tokeru.com/cgwiki/index.php?title=HoudiniDops#RBD_deintersect_the_easy_way [www.tokeru.com]

Hard to say without a hip file. Post one and I'll check it out, but if it is that you have overlapping shards, you can add an exploded noise before the sim with a low value to create a little distance between each shard, and see if that fixes the problem. Also, i'd recommend looking at what the collision representation of the shards looks like in dops. It's possible you have a padding on your geo, that is also intersecting. There are multiple things it could be, but construct a simple scene replicating the problem, if your scene is too heavy, and ill check it out.