Immigration is a positive force when immigrants are obliged to assimilate into their new host culture. This includes things like following the new cultural norms and leaving behind practices that are abhorrent to the host culture, while still bringing new ideas and perspectives. It also includes learning the local language, which is critical for assimilation.
This melding of new and old is what creates the good kind of diversity that makes a culture stronger through evolution. Like every evolutionary process, it is necessarily slow, as people take time to adjust to new norms.
Immigration without assimilation, on the other hand, creates enclaves of a foreign culture within the original. This is the bad kind of diversity. These incompatible cultures, in proximity with no geographic boundaries, lead to strife, crime, violence, and eventually cultural breakdown.
Importing too many immigrants in a short period of time - or worse, actively pandering to the foreign culture in these enclaves - removes the incentive to assimilate, virtually guaranteeing this result.