Naturally, there was speculation that this had something to do with the Michael Crabtree situation. However, the odds are pretty slim of that being the case:
Because 49ers had open roster spot, they had to promote WR Chad Hall, a third-year P-squad player, or cut him from P-squad.— Matt Maiocco (@MaioccoCSN) January 20, 2013
When a team releases a player from their roster and has an open roster spot, if they have a third year practice squad player, they have to promote that player. Players are allowed two seasons on the practice squad, but a third year can be allowed in certain stiuations. So, as Matt Maiocco pointed out, the 49ers either had to promote Hall from the practice squad or release him.
My guess is Hall will be one of the seven inactive players on Sunday. Every indication is that MIchael Crabtree will start on Sunday. The team has a host of special teams players, so Hall is simply an extra body to fill an inactive spot with much of the 2012 rookie class.