Laymen explanation
For example, if someone was typing the term Michael Jackson (musician) one week before his tragic death, Google Autocomplete would've probably suggested something like (Michael Jackson (musician) top music downloads), while if you tried to search for his name 20 minutes after his death it would've hypothetically suggested something like (Michael Jackson (musician) death latest news) due to the sudden change in search patterns related to this new topic/context, during that time frame and in relation to the breaking news.
Technical explanation
"As you type, Google's algorithm predicts and displays search queries based on other users' search activities. In addition, if you're signed in to your Google Account and have Web History enabled, you may see search queries from relevant searches that you've done in the past. All of the predicted queries that are shown in the drop-down list have been typed previously by Google users. Google uses below info while showing auto-complete suggestion.
Suggestions Based On Real Searches
Suggestions Can Vary By Region & Language
Previously Searched Suggestions
Suggestions Are Ranked
Deduplicating & Spelling Corrections
https://www.quora.com/How-does-Googles-autocomplete-feature-work