Risk of injury is a factor of the working environment, not the skill level required for the job; I'd class electrical engineering as high skill level, but it can take place in highly risky environments. Also, if an environment has an inherently higher risk of injury, it only increases the need for some form of pressure to ensure that same risk is reduced or at least noted by the employer (who is responsible for the working environment).
And skilled jobs aren't exactly immune from bad working practices, etc; unions aren't restricted to manual labour, but applicable to all jobs, as there is always a potential for abuse of employees by the employer. Whilst there are some 'bad' unions (I'm looking at you, Scotrail train drivers!), this doesn't negate the purpose behind unionisation. Just look at, for example, the guys at EA forced to work 80-hour weeks without proper overtime pay.