Each hosting account on the server is allocated a number of worker processes that process user requests. Requests go to the server and queue up. Light requests are processed quickly, and heavy problem requests are processed slowly, slowing down the progress of the queue. When the queue reaches a certain length, the server stops accepting new requests, returning the error “503 Service Temporarily Unavailable" (service is temporarily unavailable).
The most common causes of the problem and possible solutions:
Also, the reason may be a lack of memory allocated to the site for OPcache.