Damaged head (from a head strike, or head is cracked/ damaged in some other way), ink line leaking, a fitting loose, or cracked, cracked damper, internal leak in the head, something stuck on the head by the magenta that's collecting and dropping the ink (usually doesn't drop that much, but a possibility), electrical issue fully opening the magenta (not common, but it happens)... Lots of different possibilities. Clean it up, and follow it back to the source. First hunch is head is damaged, either externally from a strike, or a failure internally. If you have any puddling or pooling of ink in the capping area, losing magenta when it's idle, or ink outside of the head where it shouldn't be, check the magenta ink lines, dampers, fittings for a leak. That's a lot of ink, if you're lucky, it's just a leak, which is easier on the wallet to fix.