I would assume that the cause is due to a new execution plan being determined. I suppose it could also be a bug, but I'm assuming you are on the most current service pack and it has been stable by my experience.

A plan change could have resulted from a variety of things. It could be because the new installation uses different default settings such as a change to the ANSI_NULLS or QUOTED_IDENTIFIER settings that impact the execution plan or the comparison logic. It could be due to a lack of statistics being established yet on the new server. It could be due to a decision logic change in the plan generator. If you still have access to a SQL 7.0 version of your system I'd evaluate the plan from query analyzer in both environments and try to identify what changed. It may lead you to the source of the change.