Concept: Crowd behaviour in mass gatherings refers to actions and reactions of people in crowded places. These behaviours may include pushing, rushing, panic movement, irresponsible actions, and competition for access.
Step 1: Check crowd-related actions.
Scramble to get event tickets is a crowd behaviour because people compete for access.
Tussle to get a glimpse or autograph of a celebrity is also crowd behaviour.
Mad rush to leave a school is also a collective crowd movement.
Unruly and irresponsible crowd behaviour is directly related to crowd behaviour.
\[
A, B, C, E = \text{Crowd behaviour}
\]
Step 2: Identify what is not crowd behaviour.
Communication delays are not a behaviour of the crowd. They are a management or communication problem.
\[
D = \text{Not directly crowd behaviour}
\]
Step 3: Select the correct option.
Only D is not linked directly to crowd behaviour.
Therefore, the correct answer is D Only.