Well, ultimately, it’s a novelty song. It exaggerates because it’s funny–like the truckload of hogs stinking so bad they closed up the Duck’s sinuses from five miles downwind. Nevertheless, I’m going to try to analyze the scenario as if it were a (mostly) real account.
Insofar as logic applies, the reason for the extreme police reaction was not just because they were driving down the road together. It’s because they were doing something illegal (and potentially dangerous) and refusing to submit to authority. By the time the song first references a major police reaction (a roadblock and a police chopper), they had traveled over 1400 miles. The Duck originally claimed they were clear (of police) all the way from Los Angeles to Flagstaff, Arizona, which is only about a third of that distance. Presumably, after that point, a number of attempts were made to get them to stop, which may well have involved multiple (futile and dangerous) high-speed chases.
In Tulsa, the Duck effectively declares war on the cops, leading 85 big rigs (which would be a pretty terrifying mass of metal moving at high speeds) to crash through a roadblock. Such roadblocks are often formed of police cars, so he’s probably added destruction of public property, at least, and possibly reckless endangerment of the cops and the members of his own convoy. At this point, it would be pretty reasonable for the authorities to conclude that the convoy presents a serious threat to anything in its path. Frankly, the military escalation at Chicago (another 700 miles, at least one major city, and presumably multiple other attempts to stop the convoy down the line) seems like a pretty rational response; they’re trying to stop “a thousand screaming trucks”, at least one of which is loaded with explosives. Would you want to tackle them in a squad car?
The real oddity isn’t the police reaction, it’s why the convoy is doing this in the first place. Obviously, they aren’t just trying to complete a delivery quickly, though it seems to have started out that way. Then it escalated to (somewhat) civil disobedience, wherever a cop first tried to pull them over for speeding–they ignored him just as they were ignoring the new speed limit. Why did the Duck decide to turn it violent at Tulsa? Why did the rest of the convoy go along with it? Were they really that pissed about the speed limit? If it was really intended as a protest, drawing out a major roadblock should have garnered enough attention to get their grievances on the news.
Maybe the Duck just wasn’t rational by then. If he drove non-stop from L.A., then even at extremely high speeds, he’d have been driving for somewhere between 16 and 20 hours, and there’s no telling if he was rested when he started. So he was punchy, and probably feeling a mix of anger at the cops who’d tried to pull them over and arrogant satisfaction at blowing them off. When he saw the roadblock, he didn’t make a reasoned decision about a protest, he just got angrier, and (feeling invincible) decided to crash through it. The rest of the convoy had been following him for the better part of 24 hours, and were probably drowsy and halfway under highway hypnosis; they may not even have realized what was going on until they sped through the scattered remnants of the roadblock–at which point, they were sort of committed. (Though I note that Pig Pen quietly withdrew from the convoy at some point and went off in another direction.)
Well. That turned into quite a heap of over-analysis, didn’t it? It’s just a silly song, after all.