What is the most common cause of voltage drop?

The most common cause of voltage drop is wire that is too long or too thin for the current it carries. That is normal physics, present in every circuit, and it only becomes a problem when the run outgrows the gauge.

Abnormal drop is different, and its usual culprit is a bad connection: a loose termination, a corroded splice, an overheated receptacle stab. Resistance concentrates at one point, drops voltage there, and makes heat where you least want it.

Undersized DC systems show it worst; at 12 volts, losing 1 volt is losing 8 percent.

Diagnose by measuring voltage at the source and at the load under real current. The difference names the problem.

For battery and vehicle runs, the DC voltage drop calculator shows exactly what a given length costs you.