I've had this issue multiple times. It seems to be related to high USB I/O load; in my case to an external USB spinning disk, perhaps with network I/O simultaneously.
I've not been able to reproduce it on demand, tho; when re-starting similar traffic, it seems fine for quite a while, often longer than it takes the disk transfer and network I/O to complete.
To try to reproduce, I would try copying a big file to disk from SD; rm; loop while doing 'make build' on the disk; along with FTP of pkgsrc or something like that.
-Mike