As to the 'why'.. it can be a lot of different things
Somebody might of side loaded it during installation..or used it for a fulcrum to help pull an injector..
Somebody might of used a little too much ether..
Somebody might of used a impact gun once or twice..
Likely, though, too many times r and r'd. That whole area is a real ring a ding ding. for shock waves... and any pre stressed bolt is not going to survive under tension.
Might as well replace the other 3 while you are at it!
WW mentioned to not weld.. I too would be pretty leary around all the electronics. I've heard that disconnecting the battery, and connecting the cables together may limit any 'stray'.. I dunno. I'd at least put the welder ground as close as possible to the actual welded area!! Do not clamp it to the next injector, or any fuel lines! Maybe practice on a couple of similar sized bolts and washers to get a feel of what it's going to take.
His advice on 'smacking' the bolt downward will help jar the threads loose. I'd use a center punch, directly in the center of the broke part. One smart whack should be enough.
IF you do weld, find a steel washer just big enough inside diameter to cover the end of the broke bolt.. and tack it to the washer. Press down hard on the washer as you weld it.. otherwise the washer will simply lift as the weldment forms. Use a screwdriver or something. The washer will pretty much protect the alum. (in case you 'miss')  Then tack on a nut or something on the washer to 'grab' onto. Give it plenty of time to cool, that bolt will have expanded and may be really tight until cooled.
Rock the nut back and forth gently, until you feel it making headway.
Unless some goof cross threaded it, it should back out pretty easily. I've never been 'beat' by this method, but several attempts sometimes are reqd.
I'd even try using JB weld on the washer. Pre weld a nut to it, and flow some jbweld down the inside of the nut and it should bond to the broke nub. It's gotta be clean and dry though!! The worst that could happen is the cold weld will simply shear off. My problem with the JB is I'm not patient enough to allow full cure.
Stuff a rag into the injector bore first.. you don't want a welding splatter landing on the sealing surface, and take the rag out later so you don't have a fire going!!
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