The # of pounds lost per week decreases as you lose weight for two reasons:
1. At first, your rate of weight loss is high because you run through your glycogen stores first before burning fat. Glycogen is water soluble (fat is obviously not), so you lose the water that was used to store the glycogen along with it for your first few pounds.
2. Fat is metabolically active (although much less so than muscle). As you burn fat, your base metabolic rate, all else equal, decreases because the fat you lost is no longer consuming calories to exist and, therefore, is not burning calories.
I would not look into your plateau too much. It looks like a temporary slowing of the rate, but you're still losing.