Sending a Direct Message results in "This request looks like it might be automated"


Years ago when Twitter used to get down regularly in the day, I made my own php client hosted in my own server to keep a copy of my timeline. I still use it, but now recently I started to get the following message whenever I try to send a Direct Message to my friends:

“This request looks like it might be automated. To protect our users from spam and other malicious activity, we can’t complete this action right now. Please try again later.”

Note that this is a DM to friends I have been following for years. So, is there any way I could authenticate the request is from a user (me) and not from a bot, or what flags are you guys checking to trigger such message.