Explain this to me please

Macro line is:
/castsequence [nochanneling] Shear, Shear, Shear, Demon Spikes

The second i start combat Demon spikes goes off. Is it cause its a Instant cast OGC spell??

Debugger says:

Vengance,5,Shear, Shear, Shear, Demon Spikes,Not Able to Cast… Why does it read the whole line like it tried to fire off all actions on the line.