I can't speculate on why the gasket failed as I have no information on how you proceeded with the job and how well you adhered to standard procedures and protocol.
But, I would advise not to torque the bolts another 90°. I think that's really bad advise.
The two angle turns after the initial torque are a procedure for specially made "torque-to-yield" bolts (or stretch bolts as they are commonly called). The basic idea is that as the bolts get turned they draw the surfaces tighter and tighter. At a certain torque the bolt are designed to start to stretch, rather than continue to tighten. When all the bolts reach the range of tightness where they are now merely stretching, they have all reached the same level of tightness. This accuracy of even-ness of torque amongst the bolts represents an optimum of stability across the head to block surfaces for this critical job.
Turning them any further will not tighten them, they will merely continue to stretch until they actually begin to weaken their hold and likely break.

These are special bolts in a newer era of technology (though they've been around for a while) and aren't like head bolts from years ago. They have a specification for length after being used (stretched). They can be re-used as long as they haven't exceeded the maximum allowable length. Personally, I would use new stretch bolts every time.
Also note that the older style M103 head bolts used thick washers, the newer replacement (stretch) bolts have an integral collar and do not use washers.
One of the more important things you can do on this job is to make sure the bolt holes are VERY clean using compressed air, q-tips, and even a bottom tap to [carefully] chase the threads clean.

Do the best you can to make sure the leak isn't coming from the rear of the valve cover or somewhere else before you tear the head off again. The valve cover should be retightened, to the specified torque, at some point after the head gasket job to prevent leakage. Over tightening may actually cause leakage. The castings on M103 valve covers are known to sometimes distort among other problems and some people use sealers on the gaskets (I would avoid that if I could, personally).
Last thought would be that even if it was a head gasket, unless the leak was really bad, I probably wouldn't do the job until I started getting oil in my coolant tank. Then I'd do it as soon as possible so it didn't foul the radiator, heater core and the rest of the cooling system.