Who were the "tyrants" that Charles Martel "crushed"?

The answer lies, I believe, in the obscure end of the Merovingian epoch and is quite hard to understand from a modern perspective seeing that the geopolitical entitles involved disappeared completely under the Carolingians (at the risk of being off-topic, I still remember fondly reading these stories as a child and not understanding a word, so foreign seemed the names and places referred to, even though the history was supposed to be that of France).

So who were these tyrants? These tyrants were presumably on the one hand Plectrude, the widow of Pépin of Herstal (the father of Charles Martel, grand-father of Pépin le Bref and great-grand father of Charles the Great), on the other hand Ragenfred of Neustria and Radbod the Frisian. At the risk of repeating what you know very well, the kingdom of the Franks was then divided in a number of political entities, the two most central ones being Neustria (north-west, more or less anything between Britanny and Reims) and Austrasia (north-east, which is perhaps easy to describe in modern terms as, maybe not coincidentally, the battle grounds of the western front of WWI). In 715, Dagobert III died at the age of 16, having been the king for only 4 years without having exercised any power. For the last 4 years, the power had been in the hands of the Maire du palais, but it just so happened that Pépin de Herstal, the mayor of Austrasia, also had just died. So the situation was ripe for a bold power grab: no king, no mayor in Austrasia.

More or less everybody had a go at it. Plectrude, the widow of Pépin and the richest player involved, declared herself regent of Austrasia, and by implication of the whole Frankish kingdom. The mayor of Neustria (Ragenfred) all but invented a king (who "reigned" under the name of Chilpéric II), attacked Austrasia from the south-west and defeated the armies of Plectrude near Compiègne with the help of Radbod the Frisian attacking from the north-west. So it seemed they had won (especially as the Lombards favored their side mostly, it seems, because they did not want a woman as their ruler). But Charles Martel, who was the illegitimate son of Pépin de Herstal (hence the enmity with Plectrude) managed to gain authority on the disgruntled Austrasians (also unhappy with their female ruler) and in turned defeated all the aforementioned actors. Except Plectrude, who was utterly defeated, they did not give up though. Ragenfred sought an alliance with the powerful duchy of Aquitaine (but was defeated again), Chilpéric II actually reigned again (as a compromise, it seems, between all the factions), Radbod and his successors kept menacing Austrasia and it is only with the victory against the Saracens that Charles Martel ultimately prevailed.

Plectrude seemed to have been an exceptionally unpopular ruler (all the province of the kingdom of the Franks rebelled agains her rule, even her own), but it is hard to say whether this was mostly or entirely due to the fact that she was a woman (she is also a saint of the Catholic church). The mayors of Neustria are usually depicted as especially cruel and vicious in French historiography (the bad guys, so to speak) and there must be at least some truth to it considering how many died murdered by a lord they had wronged in some way. The fact that they were on the loosing side certainly played a role as well, of course.

So I believe these were the tyrants. Any comparison of these events with Game of Thrones is, it seems to me, unjustified: despite dragons, the latter is much more believable.

Sources:

Charles Martel is a good starting point. I did not make the effort to check the English versions of the names of the key players, so my apologies if some names are a bit off.