First off you have a couple misunderstandings about Cron Expressions. "*/20 * * * * ?" is every twenty seconds as the comment implies, but only because 60 is evenly divisible by 20. "/50 ..." is not every fifty seconds. it is seconds 0 and 50 of every minute. As another example, "/13 ..." is seconds 0, 13, 26, 39, and 52 of every minute - so between second 52 and the next minute's 0 second, there is only 8 seconds, not 13. So with */50 you'll get 50 seconds between every other firing, and 10 seconds between the others.
That however is not the cause of your rapid firing of the job. The problem is that the current second is "50" and you are scheduling the new trigger to fire on second "50", so it immediately fires. And then it is still second 50, and the job executes again, and it schedules another trigger to fire on second 50, and so on, as many times as it can during the 50th second.
You need to set the trigger's start time into the future (at least one second) or it will fire on the same second you are scheduling it, if the schedule matches the current second.
Also if you really need every "N" seconds type of schedule, I suggest SimpleTrigger rather than CronTrigger. SimpleTrigger can do "every 35 seconds" or "every 50 seconds" no problem. CronTrigger is meant for expressions like "on seconds 0, 15, 40 and 43 of minutes 15 and 45 of the 10 o'clock hour on every Monday of January".
makes sense, i changed the code of the test app and it is now behaving as i expected... thanks a lot