When P2Pool learns that there's a new block, it mines empty blocks until bitcoind gives it a block template.
The alternative to doing this would be to mine orphaned blocks rather than empty ones. :/
Check the times of the blocks:
2016-04-15 02:57:19.832312 UpdateTip: new best=0000000000000000052e336f67c1b0d339d5e7423595c60f169cd17bd24e5306 height=407354 log2_work=84.48832
tx=122495865 date=2016-04-15 02:57:07 progress=1.000000 cache=4.7MiB(5564tx)
2016-04-15 03:21:13.214595 UpdateTip: new best=000000000000000003828fbc61ad281aa2e298ed4a299abf36209a4725d51930 height=407355 log2_work=84.48836
1 tx=122495866 date=2016-04-15 03:20:49 progress=1.000000 cache=51.3MiB(16114tx)
Doesn't look like it was immediately after a new block...