some fork happend strange.
Just now, I found my delegate alt && dorian begin to miss  block. 
So I wait until next block generate.
first I see the block generate normaly, block ID is 4231.
after a minutes, when block 4232 is sync. 
I check block 4231 again, it's different with before, means dorian create  block 4231 but refuse by the main chain.
dorian  lose block 4176 && 423,  at that time,  forked occured at 4174 && 4229.
after restart the client, everything is ok now.
delegate (unlocked) >>> blockchain_get_delegate_block_stats 299
....
  ],[
    4130,{
      "missed": false,
      "latency": 0
    }
  ],[
    4176,{
      "missed": true,
      "latency": null
    }
  ],[
    4231,{
      "missed": true,
      "latency": null
    }
  ]
]
delegate (unlocked) >>> blockchain_list_forks 
....
  4174,
  4180,
  4182,
  4218,
  4227,
  4229
]