Because that was a very common weak spot on those helmet molds, even the slightest pressure of someone leaning on the piece would cause it to break like that. It's a detail that was charmingly represented well in the film.
The old plastic they used was pretty brittle. I can attest, as someone who has 30 year old "well-loved" Lego sets, I have '80's Space-men' legos that look exactly like this. Great iconic representation.
This might be the best product placement movie. I was tempted to toss money at the screen when I saw the trailer, but then I remembered that Lego has a store on the other side of the complex so I walked over after the movie.