safety is first and foremost the most deciding factor
then there are things like:
- time it takes to do the job
- available resources it would take to do the job (ie: the ability to remove any component that would require a crane or other type of lifting device a field tech wouldnt normally have on his truck)
- whether the customer wants it done in the shop or done onsite (see previous reason)

if a tech has the resources to do the job on site that is his main goal but if it is going to take a long time or require resources he just simply cant provide on site then the obvious choice is to send it to the shop. The tech usually makes the initial decision but ultimately it is finalized by his management and the customers decision.