Why introduction of an extra copy of a gene related to pigmentation causes RNA interference in Petunia?
The answer is in the slides you provided a link to. The key fact is that Craig &amp; Andy did careful controls to show that it was the presence of dsRNA in both the sense control, and in the anti-sense experimental sample that was responsible for the RNAi-mediated knock-down.  Actually, Ken Kemphues showed this earlier in a Nature paper (the control also knocked down gene expression) but did not pursue the mechanism.

Therefore, I infer that the transgenic gene expression construct introduced into the petunias must also give rise to dsRNA. I am less familiar with the consequences of introducing foreign DNA into a plant cell (e.g., does a single copy become integrated, multiple copies?, is it maintained extrachromosomally?)