I wouldn't recommend using percentages for auto growth options on such large DBs. For instance if you have a DB that's 100GB, has autogrowth set to 5%, and the time comes when it needs to grow - that's a 5GB growth that needs to occur for the data file. Depending on your disk subsystem and whether or not you're using Instant File Initialization, it may take quote some time to grow your file (which could temporarily cause your entire DB to be unresponsive - as file growth is a very IO intensive operation)

It sounds like the percentage you had it set to resulted in a large growth rate, which honestly takes time to complete. When you changed it to a mere 500MB, the growth was pretty much instantaneous resulting in your DB coming back online 🙂

IMHO you should size your DBs large enough to AVOID growth (this operation should only occur RARELY). This will reduce the growth and will save you much time and effort 😀