TRUMPF: Ask the Expert - Hairpin welding 

Hello and thanks for your question. Welding from the side would potentially be possible as well. However, heat input is lower comparing welding from top versus from the side, as in the later case we have to fully penetrate through one wire first before even melting the second one, energy flow will be asymetric. In case of welding from top side, we can evenly distrubute the energy input.

Nobody does this by hand at a factory. If these had to be welded by hand manufactures would use a different motor design.