It is without question that most people applying to incubators apply to a few of them, in case they don't get into the (obviously) best one, YC.
Its my guess that some of the less exciting incubators give these exploding term sheets to make the founder think:
"Damn, I got into Incubator X but I have to say yes in 48 hours. I wonder if I will get accepted into YC also when answers go out in 2 weeks. Should I risk walking away with nothing, or just take the offer I have now..."
Would not be surprised to see more incubators doing this anyways, with or without exploding term sheets. They will likely just move up the dates their sessions start (and the date you need to say yes by), to make sure its before YC answers go out.
On the same hand, I am not sure what you expect other incubators to do. YC is hands down the most prestigious incubator to get into and everyone knows that. So, you have a bunch of other incubators that have to think scrappy to get people into their programs. These guys don't just want YC's cast-offs, after all.
If YC was to start doing rolling acceptances for a start date, it would solve the issue of other people doing exploding term sheets. If I apply to YC today for the winter batch and can interview and get a yes in a few weeks, then the motivation for other incubators to do this behavior is gone.
YC partners need to realize they are the cause of other incubators giving out exploding term sheets. I know YC likes to make a event of doing all the interviews/acceptances on one day, but it would certainly be much more entrepreneur friendly to have it be rolling admissions. The start dates for sessions can still remain the same of course.