Does the forum logs give an error when it crashes?
Do you have mongo and nginx logs on?
There should be some useful information in one of the logs as to what is happening.
https://github.com/NodeBB/NodeBB/commit/a7bb5893e8297853cad15d104c87ad72d5ec0db2 might fix this issue, the same was happening on topics as well. If you navigated from topic to topic or from category to category, the socket listeners werent removed.