Generic Disclaimer! Transformers isn't mine, and I make no profit from this.
The story ahead is utter silliness.
Few mechs, either within or outside the Decepticon ranks, knew that Megatron's helmet wasn't really a helmet. Fewer still knew the story of what had happened, or that it was all Starscream's fault. At least, the way it ended up was Starscream's fault.
Starscream had entered the Decepticons when they were still just a small group, but his arrogance and attitude rubbed the others the wrong way. And now he'd been challenged.
"Listen, it's simple," the shorter mech growled at him. "Get this bucket on his head and get pictures. How is up to you. If you can do this, we'll respect you, but I for one think Megatron'll just turn you into a pile of scrap."
"Give me that bucket," Starscream demanded, snatching it out of the other's hands. "No one scraps me. I'll do it."
And so it was that at a party one night, Starscream oh-so-subtly made sure that Megatron had a little too much high-grade. Okay, maybe a lot too much. Then, he made sure he was the one who helped Megatron weave and stumble his way back to his quarters. The instant he stepped through his door, Megatron offlined, and would have fallen flat on his face if Starscream hadn't caught him. Starscream jammed the bucket on his leader's head and snapped a couple of pictures.
Then, he got An Idea. A horrible idea.
He was going to one-up those fools who thought he'd be scrapped. He dropped Megatron on the floor and ran off. Returning with tools in hand, he made sure Megatron was out cold, and proceeded to weld the bucket on.
They never could get the bucket off, and were eventually forced to just cut a hole for Megatron's face.
The other mechs still didn't respect Starscream, but they never ratted him out, either.
Starscream quietly arranged for the unfortunate demise of everyone who knew he'd done it.
Megatron didn't remember a thing about how the bucket had gotten there, or indeed the entire night, and told everyone it was a helmet, if they knew what was good for them.
Short & sweet, thanks for reading! - Illogically