What version of Impala are you running? How big is the cluster?
Are the queries closed and unregistered? We've seen cases where some clients leave queries open.
The first place I usually look to diagnose a memory issue is the /memz?detailed=true page on the Impala debug web UI, that has a lot of useful information. E.g. https://impala-daemon-host:25000/memz?detailed=true