It's so weird that it's almost too silly. Who comes up with this shit? Well, nobody. That chest-thumping ritual is something McConaughey does on his own, in real life. DiCaprio saw him doing it on the set and, rather than slowly backing away like most people would, asked him what was up. Apparently it's what McConaughey does to relax and get his voice ready before scenes, and DiCaprio thought it was so weird, he asked to do a take of the scene with McConaughey's little ritual in it.
Pictured: The exact moment DiCaprio realized that McConaughey was off his meds.