In the movie, Lawrence III wanted to capture Lugia. In order to accomplish his goal, he set out to enrage and capture the legendary birds. By doing this, Lugia was prophesied to appear to calm them. His plan was then to capture Lugia for his "collection".
Although he ended up injuring it, Lawrence III did not succeed in capturing Lugia. With the help of Ash, Lugia was able to stop Lawrence III and calm the birds before the unbalance caused the world's destruction.
Lugia communicated with humans through telepathy, much like Mewtwo did.