Originally Posted by BikeMax
It seemed to follow a change made out of the saddle but not immediately - maybe a coincidence.
Can you elaborate on "the change", e.g., was this a rear derailleur shift?
In general, if you threw the rear chain off the big ring only one of a few things could have happened:
Work Around: If the chain derails followed a shift to your smallest cog, make sure your FD cage is parallel with the big ring and the outer plates is within a millimeter of the chain in the big/small combination in the stand. Yes, it will probably rub once you're actually riding but if you find you're not dropping the chain you can move the stop screw out a 1/4 turn at a time until the rub is gone and/or until it starts to drop again. Chain rub in the big/small may be the lessor of two evils.
Root Cause: Some thoughts on what could be going on here.
1. The bottom bracket is deflecting under load: make sure it's tight.
2. Your big ring and/or chain is worn-out and the chain is slipping off. Consider getting a new chain / chain ring if it's got a lot of miles.
3. The crank and chain ring are deflecting under load. Spin higher revs or get stiffer cranks / crank system.
4. The rear triangle is deflecting under load: spin higher revs.
As for sprinting on a tandem, it's all about working well together and keeping the revs up. Climbing hills out of the saddle is great training for sprinting since the movements are the same and, again, higher revs will always yield better performance and reduce bike problems in both situations.