We only know each other by reputation so I hope you don't take it personally, but I'm poaching your client. I ran into them at a recent industry event, and after asking a few questions I learned that they have been working with your firm for more than five years. During the first few years, they were very happy with your services. You eagerly learned as much as you could about their needs and provided numerous value-adds to keep them happy.
Lately however, they feel like you have gone on autopilot. They no longer receive any special attention from you (unless their invoice is overdue), and you have not been proactive about the needs surrounding their latest B2B product (sorry if this is the first time you're hearing about it). They recognize that they're not your biggest client, and they value the relationship they have with you, which is why they have not actively been looking to replace you. But once they heard about the introductory discount, needs assessment, and value-add I was willing to offer, they decided that they were better off making a change.
This will come as a shock to you, since your last round of client feedback with them was in 2014. You've probably been coasting along with the thought that no news is good news. I hate to say it, but you made it easy for me. I look forward to meeting more of your clients very soon.