With the caveat of this being uncorroborated altitude data, something like this could be a TCAS RA, (unlikely, so close to take-off) or dipping under birds. I have done the latter twice on take-off, although, I didn't descend, I just levelled out to fly under the crossing birds, and resumed climb once they had passed.
I did have to increase descent rate to stay clear of a banner towing aircraft that had no transponder and ATC did not know about, that I saw crossing our path on initial approach to Alicante once.