The bark "slips" in spring/early summer on most trees. The cambium is dividing to make wood and bark and is easily removed. You can often roll a log right out of its skin. To tie back into the time of harvest thread, this period is when the earlywood portion of a years growth ring happens, in about a month's time the lighter, less dense ring and the years' growth spurt happens. Hard to keep a tight jacket with all that going on.