Once you have a myth associated with something, as the witchcraft connection of this play would do quite happily in the first century of the plays life, people become nervous around it. A nervous stage-hand is that bit more likley to drop a back drop on someone. Now the curse has caused an accident, so it's believed in that bit more. Positive feedback....

If there's something big in Loch Ness, then there's something in plenty of other lochs, but people are only wanting or expecting to see something in the one. You get a self re-inforcing myth.

As noted above, people will also remember the accidents more so than for other plays.