A little help with dried out cork

I have a few nice bottles of Scotch that I've kept upright for about 4-7 years. I recently opened up a bottle only to find that the cork came apart in the bottle opening. I opened up a second and the same thing occured. These bottles are stored in a cool / dark location.

I'm a bit concerned about the other bottles and would like some advice on whether or not these bottles should be kept on their side to help keep the corks moist? I've never believed that a cork could dry out such as this one has, but now I'm second guessing. I've had many a bottle of wine withstand similar durations upright with no issues whatsoever.

Any other thoughts or suggestions would be appreceiated! Thanks so much.

Drew