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Why is my bread now deflating when I slash vents before baking?

I make bread once a week from the same sourdough starter, which lives in the fridge between feedings.  On the day before baking day I take it out, feed it, wait until it's fully active, take some of *that* for the *levain*, and put the rest back in the fridge.

For the last two weeks, my dough has risen overnight as expected, and then again after shaping, but when I've gone to slash the vents right before baking, both times it deflated somewhat (about 15-20% I'd guess).  It then rose back up in the oven ("oven spring"), but unevenly -- the top was a little lumpy from the unevenness caused by the deflating.  I've never seen this behavior before.  The resulting bread tasted fine and had about the amount of crumb I'm used to (maybe a *little* finer, but within normal bounds).

I usually bake white breads, though I've made rye and whole wheat loaves before.  Both of these were rye -- two different recipes, one new and one that I've made before.  I don't know if the type of bread is relevant.

I'm not aware of any significant environmental changes.  The weather has gotten warmer here during this time, but I baked bread last summer too and we're not yet into the *very* hot part of the year.

What might be causing this new behavior, and what should I do to correct it?  I'm open to experiments, though I'd like to keep the rate of total baking about the same as it is now.