The cause of the issue is most likely a memory-related one. Although you’re opening a small document, all documents are loaded into memory for tag count, tag autocomplete, and Search Everywhere to work properly. When an app uses over 100 MB of memory, iOS kills the process and hence the reloading issue.
I can try identify which are the biggest documents in your account, and the issue should go away if you export them and then delete them. In the previous case we looked into, the person had a few full books and a copy of Bible in his Dynalist, which caused the out of memory issue.