Was the thread old - or a decent brand? I'm not pushing any particular brand - but I watched a Bernina 830 go top speed with intensive Christmas Lace ornaments. I watched that machine for a half hour at least. Thread never broke. I bought the brand, think it was YMNET, used it for the same ornaments in my Pfaff 2170. No breaks. I acquired some silver thread from Babylock that behaved the same way. Both were larger spools. I've had trouble with Sulky. Especially old spools.
The other thing would be to make sure the spool of thread is upright and winding off the spool in the right direction. Use a thread net too if it's needed. Most machines have some kind of adapter for a vertical spool. Brother and Babylock had adapters that went over the bobbin winder, Pfaff has an upright spool holder that fits into a hole in the top of the machine.