Just assume your competitor is cold calling all your existing customers. They should be. Yes, sometimes the yield here can be low if your customers are very happy. But it is worth a shot if you have the resources and are in a competitive space.
Second, assume your competitor has a database of when your contracts are up for renewals, and does campaigns around those dates. They should.
Third, assume your competitor reaches out aggressively to your customers if you have downtime, a security issue, etc. Some will use that as a reason to switch.
Fourth, assume your competitor may offer big discounts for switching — bigger ones that they’d usually offer. And even buy-out deals. They may be willing to subsidize the cost of switching now, even if they are on annual+ contracts.
Assume it all. When you are big enough, you may (or may not) want to do the same. The ROI here often isn’t huge, but it is real. Once you have enough resources, it can make sense to assign some here.
And finally, assume your competitor continues to market to all the customers and deals it has lost to you. This is so easy — they already have the contact information and deal basics. It may take 2–3 years to win a lost deal back. But they likely will get another shot at it, especially if you don’t treat your customers incredibly well.