That is a good question. I don’t know every case there is but I‘ll use Spain as an example for something like this happening. Prehistory: After the Carthage lost Sicily, Sardinia and Corsica to the Romans 241 BC (end of first Punic war), they focused on securing their Spanish colonies, especially the Barcides (Hamilcar and Hannibal Barca). Alliances were secured by either conquering Spanish tribes or marrying family members (like Hannibal) to Spanish princesses. Subsequently Hannibal was seen by Spanish tribes as a prince/ king. After he started his campaign towards Italy, Rome sent a consul (Publius Cornelius scipio) and his brother Cornelius Scipio to Spain, but both got killed 211 BC by Carthaginian forces stationed in Spain. After that Publius Scipios son, Publius Cornelius scipio (later africanus) is sent and in a surprise attack 209 BC he takes the Carthaginian capital in Spain: Carthago nova.

This was a very important victory for the war effort against Carthage. Not only did Carthago nova contain Carthage‘s warchest but also lots of grain, ships and most importantly: Spanish prisoners and hostages who scipio sent back to their tribes. Thus, the war in Spain shifted in Rome’s favour, since several tribes, even Carthage’s closest allies the Ilergetes, changed sides to the Romans and provided troops (which Rome was in dire need of). Not only that, the chiefs of the Ilergetes also proclaimed scipio as a King (and several tribes did the same after he kicked Carthage from Spain once and for all after the battle of Ilipa 206 BC) But he denied the „honour“ because it was a political danger. Rome disliked tyrants since it got rid of its kings (Rome’s kings are an entirely different and much debated story and I don’t want to get into that but: for the Romans it was important that they had no king who held all the power and authority). But he wanted to be called something like a „supreme commander“. The trend of Spaniards calling Roman commanders kings did not last however after it was clear that the Romans planned on staying. The reason why they probably proclaimed him as king was that they hoped he would free them from the rule of the Carthaginians. As soon as he fell ill and the rumor spread that he was dying, the same ilergetes who proclaimed him as king revolted but were quickly put down.

Since this is one of the few instances that I know for certain where a commander was proclaimed king, it could probably have happened a lot more when a Roman commander showed his skill in battle. Sadly I have trouble getting good literature on Celtic culture but it’s apparent from this example that some Romans were proclaimed kings when they were seen as strong and skilled liberators. However, especially in the republican era it was dangerous to accept the title of king and consuls really had to pay attention that word didn’t get out to the senate. Their worries also weren’t unreasonable. The historian A. Schulten claims that Scipio could have used his Spanish conquest, years of fighting side by side with his legions and the Spanish tribes proclaiming him king as a basis for overthrowing the republic but didn’t since he believed in the republic (I‘m sceptical regarding this hypothesis but the points he made are not that bad). We see something similar with the Sertorius revolt who also used Spain as a basis for his war (and I think he was also proclaimed king by several tribes as well).

I hope that helps a bit why they some could be seen as kings and what was problematic about it. Roman commanders could also seen differently by different cultures of course but that topic would be too big for me to get into. I don’t have my sources with me at the moment but a few that I remember are A. Schultens works about Numantia and Martin Luiks „der schwierige Weg zur Weltmacht“. I can give you more sources later if you like but some are German.