I don't know what causes this but have seen (and repaired) it before. It's not hard to repair because that's a thick piece of plate. But this hasn't been done all that well. It looks from the first shot like the weld doesn't go all the way through. You need to cut a bevel and fill it with weld in order to make the join go across the whole thickness. It's also a bit ugly with too much build up (was probably MIG or flux core) that he hasn't ground down. But this is mainly an aesthetic problem.
It probably won't fail soon though and when it does it shouldn't be all that dangerous or dramatic.
It could easily be ground out and welded better because there's plenty of metal there.