This was posted in a parents support group and honestly it explains it so well.
I have been through this exact same thing with my son, but thankfully ‘tray fall’ days have becomes less frequent.

FYI– about students with Autism/ Asperger’s
by Lenore Gould (1996), Carbon-Lehigh Right to Education Task Force,
Schnecksville, PA
Distributed by Autism Services Center, Huntington, WV, by kind permission of the author

Those of us who have daily contact with children with autism sometimes have trouble explaining to regular education teachers or administrators the “hooks” of autism; especially the kinds of support they need. You’re always trying to explain the basics, “no, moving the pencil sharpener’s location in the classroom is not what upset him. You have to understand that…” Then I came up with this analogy. Perhaps it will help others to visualize the support needed.

Try to imagine the child balancing a large serving tray on one upturned hand. Every distress for that child is like a liquid-filled glass you are putting on this tray. The “distress glasses” are unique to each kid; but generally include things like auditory or visual over-stimulation, social interaction, “surprises” or unexpected changes in the schedule, lack of clear leadership, the number of people in the room; whatever is sensitive for that child. (Don’t forget the ability to read the body English and anxiety of the adults around them!) The size and weight of the “glass” for that child varies. Some things are merely shot glass size, while others can be a 2-liter jug. At some point the tray is going to start to wobble– the liquid will start spilling out of the glasses on the tray. The cues that this is happening will vary from kid to kid: just as the cause and size of a “glass” varies kid to kid, but will generally include regressive behavior, avoidance or shutting down, giggling or minor acting out to get attention.

Hopefully, someone will help the kid rebalance the tray, or remove some glasses. Perhaps taking a break, or allowing time to refocus or process will work; again, techniques are unique to each kid. If there’s no intervention, the addition of one more “glass” will topple the tray to the floor. The cause is not the most recent “glass” you added, but the fact that the tray was full or too heavy (the latter is why the child seems so unpredictable to some people.) Our efforts should be that the kid learns to hold a bigger tray, or to do minor correction of the tray’s balance somewhat independently, but they will always carry that wobbling tray. Ignoring cues can be disastrous, from classroom disruptions to a major regression. When an autistic kid’s tray comes crashing to the floor, it is always a major event.

That’s why, if I hear my son got highly upset over a moved pencil sharpener and acted out, I do not want to hear that he has to learn to accept change. The sharpener is immaterial, if I learn that day he’d dealt with a substitute teacher, a fire drill just as Reading was starting, dead calculator batteries halfway through math, a crashed computer in the middle of English, a late bus so that he missed part of home room and some fluorescent lights in the class are half out- his tray was already fill.

All of the distresses are unavoidable and he’d dealt with them without a hitch; but each was another glass on his tray.

Autistic kids need someone around who is familiar with them; to sense how full the tray is getting and read the cues, so there’s intervention before that wobbling tray topples to the floor. that is why the type of support for these kids is critical, not just a “hot body” nearby– but the “right hot body” whom they can trust will help balance and who knows the “hooks”.