The lions hunt in a group to kill the giraffe
Sometimes the hunt fails - see second video
If a giraffe can be panicked (and they can be) then there is a good chance of a trip and fall - dinner is served.
It's not stupid if it's successful, and they wouldn't try it if they didn't know they had a chance of success. A big prey animal means a big meal, and is worth the risk of attacking it.
Until they are knocked down
Are lions retarded? Dont they see that giraffes are giant compared to them?