Unexpectedly rejected merge

[Gordon Tyler]

I received a rejected merge error in a situation where I wasn't expecting it. ASCII art follows:

* b88262c (HEAD, origin/dev/nexus-139, dev/nexus-139)
*   7aad65f Merge branch 'dev/protocol' into dev/nexus-139
|\
| * db168ca (origin/dev/protocol, dev/protocol) 
| * 61dbdb9 
| * 145d7a0 
* | 8fe9ac8 
* |   dab8f76 Merge branch 'dev/protocol' into dev/nexus-139 (*** REJECTED ***)
|\ \
| |/
| * b4c51bc 
| * 46ef322 
| * cf3e4af 
| * 2a696c5 
* | 739eda3 
* | da29245 
* |   39cdf20 Merge branch 'dev/protocol' into dev/nexus-139
|\ \
| |/
| * d974909 
. .
. .
. .
| * 05a3697 Created branch dev/protocol.
* 42bb4e7 (original origin/dev/nexus-139 before push)

Both dev/nexus-139 and dev/protocol are feature branches of master. There have been a few merges from master into both. I merged dev/protocol into dev/nexus-139 a few times with some other commits in between before pushing dev/nexus-139 to Stash. Merge commit dab8f76 was rejected but I don't understand why?

Also, the rejection doesn't appear to have actually rejected the commits. All commits look correct in Stash and my local repo.