Mysql REPLACE INTO query for Multiple rows insertion
You should avoid using triggers in conjunction with REPLACE INTO on multiple rows. Why ?

REPLACE INTO is nothing more than a mechanical DELETE and INSERT. It can incite mysqld to address deadlocks (See my answer to How I prevent deadlock occurrence in my application?)

Here are two comments that shows the LIFO approach to processing triggers


  Posted by Atif Ghaffar on September 23 2007 9:12pm    [Delete] [Edit]
  PLEASE Note that the REPLACE does a DELETE operation.
  
  We did not realize this and had the triggers that should be triggered
  on DELETE triggered.
  
  After checking all the code, we just found a script that does a
  replace to refresh the values of some fields.
  
  We should have had used "insert into ... on duplicate update" syntax
  instead.
  
  Posted by J Mike on May 4 2009 11:06pm    [Delete] [Edit] If you are
  using REPLACE INTO... triggers are fired in this order (if delete of
  duplcate key is used):
  
  
  before insert
  before delete
  after delete
  after insert
  


Your problem ? An UPDATE trigger will never fire off if this has to be done multiple times in a single operation. At least, I don't see that happening.

SUGGESTIONS


You may want to use INSERT ... ON DUPLICATE KEY UPDATE (See my answer to Update one table from another table while sorting that table based on one column) 
Stick to doing REPLACE on one row at a time