Blizzard has posted on the forums about a new memory leak happening with DirectX 11 and Mac users since Patch 10.2. Blizzard will be patching the issue with the next weekly reset.
In the meantime, Windows players can use DirectX 12 or DirectX 11 Legacy.
You can read more about how to make this change in this support article.
Swapping DirectX Engine for WoW Support Article
Mac players don't have an easy solution. The only way to fix it is to enter the following command every 30m or 1h to prevent any client crashes.
/console gxrestart
We’ve identified and are working on a fix for unbounded memory growth that’s affecting Windows users using DX11 and all Mac users.
We should ideally have a hotfix build deployed early next week with the fix.
In the meantime, typing /console gxrestart in chat should cause the client to jump back down to normal memory usage (but will keep growing again after).
On Windows, using DX12 or DX11 Legacy backends should avoid the bug all together.
For those curious about the code side of things, the bug was an
std::deque::clear() call that was incorrectly placed in a code block that is only compiled in for internal clients.
Read more on wowhead.com