I've had similar experiences on motorways whilst towing a caravan. Been in lane 1 travelling between 55 and 60 depending on the terrain, where on a downhill gradient, a HGV artic that was matching my speed on the flat behind me, decides to pull out for a slow overtake, but is unable to complete it before the bottom of the descent. The artic is then alongside my car and caravan, but the speed of the artic then drops, but the left indicator comes on, and the driver then expects me to either accelerate or reduce speed to allow him to move back into lane 1.
I appreciate the HGV driver wants to maintain their momentum, but doing it at the risk of not being able to safely complete the overtake before the end of the descent, therefore putting myself in a potentially dangerous situation, and then having the cheek to blast their horn as if it's my fault, using their weight and mass to try and intimidate me into letting them back in - to repeat Andy....tossers.
And I got my HGV Class 1 in the 80's....