There are several possible reasons.
1) The people most likely to join your community are already members (or decided not to join). Like a virus, people are either infected or ‘immune’ to it. Those left to reach are either new to the topic or in tricky places (language barriers, technology barriers, less interested in the topic). You see this in the community lifecycle.
2) You changed something about your website or promotion. You made a tweak that affected how many people visit your community. This is either a technical tweak on the community site that hurt your search traffic or a change in the service or company website which now sends less traffic to the community. Give this up to six months. If nothing has changed, you need to make another tweak.
3) The community experience has gotten worse. The community experience has worsened. You might not have updated your filter system as you grew, the quality of responses has declined, or the community is less welcoming. Too few newcomers are sticking around.
4) Most topics have already been covered. You’re the victim of your success. Most of the common questions have already been asked. Members can find the answer without having to ask the question.
5) Fewer people are interested in the topic. The overall popularity of the topic is in decline. Less people are searching for relevant search terms or your customer-base is shrinking. New competitors might have emerged where people can have a similar experience.
Growth in engagement will naturally slow over time. That’s to be expected and isn’t a problem.
The danger begins when slowing growth precipitates a perpetual decline. When you’re not attracting enough newcomers to replace the community’s natural churn, you need to take action.