It depends on how you are using Alfresco and how it has been configured. E.g. if your Alfresco receives millions of calls per hour or runs significant internal (background) processes, and you have configured the connection pool to frequently evict idle connections (default: every 10 minutes connections that have been idle for 30 min are evicted), then this could be expected. Similarily, if connections are dropped because of network issues or becoming invalid in some way, they would be dropped and recreated if needed.
Relevant config properties that affect eviction:
# number of connections to test per eviction run - "-n" = fractional test of 1/n of all idle
Relevant config properties that affect connection validation:
The above default means that when connections are retrieved from the pool, they are first validated IF a query is set for validation (e.g. I have seen SELECT 1 FROM DUAL when using Oracle DB). If the connection is valid, it will be used, if not, it will be removed and a new one retrieved from the pool. If there are sufficient idle connections in the pool, one of them is likely to be used, otherwise a new connection may be created.