They first became an enemy when they betrayed him after they blew his cover in the Amnesia Custom Story: Abduction he even found monsters talking to a pile of barrels (Additionally, this is the same Custom Story in which Stephano is born-- and dropped in a tub of acid for leading him into a Bro). From then on they try to discourage PewDie, hide stuff (usually important) and make sure he doesn't survive the game. There was even once when they collaborated with the Bro to kill PewDiePie. The only time they were ever allies is when PewDie plays Lucius and they plan to kill everyone and even at one point they help him in Heavy Rain and The Walking Dead, possibly being a rogue barrel or a spy barrel. Nonetheless, the barrels are still an infamous enemy. Due to this, most bros choose to hate barrels because of PewDiePie's hatred for them, and often nickname PewDiePie's haters as barrels or dismiss spam/rude comments, with "A barrel has breached security".
According to a level made in Happy Wheels, Bros and Barrels lived together in harmony, until the Barrels started to over-populate and began taking over the world.
Often in the games he plays, PewDie blames the barrels in an area for his death and/or injury, and sometimes he will not notice barrels at first, but will notice them after he was injured/killed. An earsplitting shriek of 'BARRELS!' will most likely be emitted after this realization. It may not seem like the barrels actually had anything to do with the injury/death, but don't let that fool you. Barrels are evil.
A barrel briefly accompanied PewDie during the final part of his Gary Dark Secrets Chapter 1 playthrough, where PewDie planned to blow it up with explosives along with some rubble blocking him. However he forgot to put the barrel there, and it survived, so he carried it with him. However, despite coming with PewDie, the barrel displayed its evil tendencies as usual, saying PewDie would die and that the barrels would take over the world. Sadly it was not brought to justice as PewDie dropped it to run from a Brute.
The Barrels are ruled by the Emperor Barrel, who PewDie kills in an Amnesia Custom Story.
Some fans speculate that the well known singer Justin Bieber, may be the King of the Barrels. Due to PewDie's subscriber count being more than Justin Bieber's YouTube account, some think that the Bro Army has defeated the King Barrel.
Explosive Barrels actually helped PewDie in his Alice: The Madness Returns walkthrough by blowing up skeleton sharks and some other enemies. In mid to late 2013, he started to see barrels not as enemies, but something that might help him, as said before, and as evidenced in BADASS CLEMENTINE!
In many video games, if you shoot a red barrel, it explode, which causes hatred for barrels from many gamers, not just PewDiePie.