What causes Concurrent Apex limit error in Salesforce?

The multi tenant Force.com platform uses governor limits to ensure that system resources are available to all customers and to prevent any one customer from monopolizing them. If a governor limit is exceeded, the associated execution governor limit issues a runtime exception that cannot be handled. When you design your applications, you can help plan for their growth by keeping these limits in mind.

One of the limits customers frequently reach is the concurrent request limit. Once a synchronous Apex request runs longer than 5 seconds, it begins counting against this limit. Each organisation is allowed 10 concurrent long-running requests. If the limit is reached, any new synchronous Apex request results in a runtime exception. This behavior occurs until the organization’s requests are below the limit.

Ultimately, this limit is in place to protect the user experience. Once the limit is reached, new synchronous Apex requests are denied. This behaviour can be disruptive to your work.