What made them drop depth charges?

[Col7777]

This may have been asked before so forgive me.
I was playing a mission last night, reckless I know but I took on a DD that was heading strait for me, I fired and got it, then I notice two more further away but turning towards me.
So I decided to turn to port, then changed my mind and turned to starboard rigged for silent and went deep.
By the time the DD's got to where I sank the other one I was a good distance away but they circled that area looking for me, I was pleased I evaded them and kept going, I was under a thermal as well so I felt even more safe.

Now these two DD's went further away to the direction I first took and started dropping depth charges, so was it my first reaction to go to port that made them think that was where I had gone, and the main question what made them drop charges because I thought they dropped them on a sound contact or was it just lets drop some in case?

I managed to turn back to port further away and luckily got lined up with the convoy, I went to periscope and stopped all engines, I waited then managed to sink 3 more ships then decided to turn back to starboard and go deep again.
The end result was I failed the mission because I didn't sink all the convoy, but I thought I did a good job and escaped without any damage, if I had stayed I may have been toast because there were other escorts with the convoy.