Why do anchors only come up after midnight?

You see that big red power boat in the photo above? I must say it is much scarrier looking at midnight when it is dragging anchor and coming at you. I'll back up a bit. Chris, Park, Coco and I headed out Saturday morning to meet up with the Windsors who were bringing their new boat home from Fort Pierce. The plan was to meet at Marker 21. The wind blew from the south at about 30 knots most of the day, so we motored all the way. We have a good dodger/canvas, so really, it was not a bad day at all. Me and Chris listened to tunes in the cockpit and Park/Coco watched movies in our cabin. We got to Marker 21 just before 5pm, and saw Imagine tucked in behind the island. She looked even prettier in person that in the pictures.
We got LaLeLu anchored, situated, and headed over to say hi to the Windsors. The island gave us good protection from the wind, and we had a very enjoyable evening complete with lightening displays passing by on either side of us. It was very entertaining watching the magnificent lightening in the distance, while chatting with our buddies and drinking a rum/coke. When we went to bed, it was completely calm and still. Of course, as all sailors know, the wind is mischevious and enjoys playing a good trick whenever the opportunity presents. At about midnight, we were awakened by the unmistakable low howl of a thunder cell. The dodger was shaking and the bow was bouncing and slapping. Chris went up into the cockpit and I heard him yell "Kenny!" Next thing I knew, he was zipping off in the dingy and I saw Imagine slipping sideways over towards the Island. I watched from our cockpit as Imagine got resituated, while the big red power boat made several unsuccessful attempts to do the same. It never could get reset and finally ended up motoring and waiting it out in the channel. By morning, the big red boat was back in the anchorage. It was a rough night - but no damage done and by morning, we had all earned a new badge. The trip back home was suprisingly calm. We tried a few times to sail and hoisted the headsail. The wind was pitifully light, so it was a day of motoring again. Our cat Coco took it all in stride - here's a picture of her taken during one of her many cat naps.