Given the fact that the officer made a u-turn in order to speedily follow her car way before the "failure to signal" happened, it seems to me watching the video without pretty much any context, that the lane change was merely an excuse to pull her over.
Any police officers knows if they really need such an excuse, they just need to follow a given car for a few minutes: it's guaranteed something will happen which justifies the traffic stop.
> Any police officers knows if they really need such an excuse, they just need to follow a given car for a few minutes
And the worst thing is they tail you when they follow you, making you nervous and more likely to make a mistake. That's also what the officer did here.
Tailgating is illegal (at least, here in the UK) precisely because it is so dangerous, for many reasons including the one you point out. Of course, that doesn't apply to officers of the law.
Illegal here in the US as well. Doesn't stop cops though. Seriously, this must be standard operating procedure for cops, happens to me all the time (and I'm white, fwiw). It usually doesn't result in me getting pulled over, but it's nerve-racking. Which I imagine is intended...
Tailgating does apply to officers of the law, they need to act in a way that reduces the possibility of serious harm proportional to any criminals being pursued. Police on duty can be guilty of DWDC&A or dangerous driving.
Any police officers knows if they really need such an excuse, they just need to follow a given car for a few minutes: it's guaranteed something will happen which justifies the traffic stop.