How not to use a crane to lift a trailer

That is not true. The boom has all the power it needs under full load to boom back. I used to operate a 70 ton Manitowoc truck crane in an earlier lifetime. The determining factor is how far from the center of the machine the load is. Simple physics and calculations. No guessing allowed.
Example: The capacity of the 70 ton crane I referenced above is 12' away from the center of the machine. In other words, immediately next to it, boom almost straight up. If I were to boom out a 110' long boom to a 45 degree angle, say load is 40 feet from the center of the machine, the capacity of the crane would only be about 5 tons. Boom down to a "flat stick" as they call it and it can barely pickup the weight of the boom, it has the power, but the limits of the outriggers allow it to tip. Some cranes with very long booms, like used in wind turbine construction are so long and heavy they have to have another crane help them pick up a flat boom until it gets to a higher angle.
All of this information is in the owners manual, and frequently on permanent charts in the cab of the crane. In addition newer cranes have all sorts of alarms to prevent this sort of thing.

 

TIIIMMMMMM.....BANG- BER!!!

 

I pity the rigger who had to attach the cables.

 

It probably wasn't that difficult. A working platform with 4' tall sides suspended from the hook would allow the workers almost complete access. Kind of fun, great views.