This appears to be normal spring leaf shed; older leaves on evergreens eventually die off as new growth replaces them over the years. When shedding, they often turn yellow before dropping. Spring leaf drop in some hollies has been observed to be quite drastic in some years, perhaps due to stresses from prior growing seasons. Normal leaf drop finishes around early summer as the new growth is maturing.

Mulch has several benefits, but be sure to not have it come all the way up to the plant's trunk. It looks here to be a bit deep against the trunk; pull away the chips so the entire trunk is free from mulch down to the ground. Ideally, the root flare should be exposed so you know the plant is at the proper depth for root health. The root flare is where the roots start to branch off of the trunk and should be just at the soil surface. Buried root flares can indicate too deep of a planting depth and can contribute to root suffocation in wet or compacted soils. (Root death can also lead to leaf drop, so it is helpful to reduce this other risk.)

Miri